OBSERVATIONS UPON THE DEVELOPMENT OF THE SKULL 
IN NEOTOMA FUSCIPES; A CONTRIBUTION TO THE 
MORPHOLOGY OF THE RODENTIA. 14 
[1889. 
PROCEEDINGS OF THE ACADEMY OF 
OBSERVATIONS UPON THE DEVELOPMENT OF THE SKULL 
IN NEOTOMA FUSCIPES ; A CONTRIBUTION TO THE 
MORPHOLOGY OF THE RODENTIA 
BY R. W. SHUFELDT, C. M. Z. S. 
While collecting in the vicinity of San Bernardino, California, 
during the spring of 1887, Mr. F. Stephens secured a female speci- 
men of Neotoma fustipes which was far advanced in pregnancy at 
the time of capture. 
From this source I obtained an embryo of this wood rat, and am 
under great obligations to Mr. Stephens for the opportunity it has 
given me to record something in reference to its developmental anat- 
omy, and my observations upon it will form the subject of the pres- 
ent paper. 
Neotoma fustipes is by no means a common species, being restricted 
in its geographical range to certain parts of the Pacific coast and 
northern Mexico. I have never personally taken this represen- 
tative of the genus, but I have on numerous occasions secured its 
near relative, the form found throughout New Mexico, Neotoma 
cinerea, of which I have a series of skeletons at my hand, of my own 
preparing. 
Just prior to birth, the embryo of N fustipes is quite hairless, and 
measures in its total length about 4cms. between vertex and tip of 
tail, presenting all the general characteristics of the embryo of this 
genus of the Muridae. For its total length, the head measures 1 
centimetre and some few millimetres and in drawing my plates 
which illustrate this account of its skull, I have enlarged these 
parts six diameters. 
In working out the skull of this little rodent I have had to make 
constant reference to the following works, which, though the only 
ones available to me at the time, by no means pretend to be a com- 
plete bibliography of the subject, while, on the other hand, they 
constitute the best aids we possess to embryological research upon 
the mammalia: 
Allen, Harrison, M. I). On a revision of the Ethmoid Bone 
in the Mammalia. Bull. Mus. Comp. Anat. Harvard 
College, Cambridge, Mass. Nov. 1882, pls. 1-7, pp. 135- 
164. 
Balfour, F. M. A Treatise on Comparative Embryology. 
London, 1881. 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
15 
Flower, W. H. An Introduction to the Osteology of the 
Mammalia, 1885. Article, " Mammalia " Encyc. Brit. 
9th Edition, 1883. 
Forbes, W. A. Coll. Scientific Memoirs. 
Garrod, A. H. Coll. Scientific Memoirs. 
Huxley, T. H. Elements of Comparative Anatomy. London. 
1864. A Manual of the Anatomy of Vertebrated Animals, 
London, 1871. 
Owen, Sir Richard. Comparative Anatomy and Physiology 
of Vertebrates. 
Parker, W. K. The Morphology of the Skull. 1887. 
On the Structure and Development of the Skull in the 
Mammalia. Phil. Trans, of the Royal Soc. 1885. 
Parker, W. N. Wiedersheim's Comparative Anatomy of Ver- 
tebrates. 1886. 
Parker, T. J. Zootomy. London, 1884. 
Besides numerous special memoirs of other authors and laborers 
in similar fields. 
A general survey of the Skull and of the investing bones. 
Professor Parker, I think, would say that the embryo before me 
was in the " fourth stage," inasmuch as endostosis has at many points 
invaded the cartilaginous parts, while ossification is more than ap- 
parent in such investing bones of the primordial skull as the inter- 
parietal and the supraoccipital, which latter is as yet in two distinct 
lateral moieties. 
We might describe the form of the skull of this embryo as being 
subconoid, the apex being represented by the snout, and the hemi- 
spherical base by the region of the cranial vault and infraoccipital 
parts. 
Judging from the skull of an adult of another species, Neotoma 
cinerea, this form materially changes by the time the animal arrives 
at maturity, for then the skull is quite flat for its entire superior 
aspect, and from incisor teeth to occipital condyles. Most of the 
structures of the basis cranii are to be found in a plane parallel to 
this superior surface. Then, too, frontal, parietal, and interparietal 
bones do not quite fulfil in the skull of the adult what they promise 
in the embryo; by this I mean that as growth proceeds the mesial por- 
tions gain area over the first and last mentioned elements, and come 
to be the chief agents in forming the cranial roof. [1889. 
16 
In our embryo the nasals have advanced to no inconsiderable 
degree of calcification after the manner of the membrane bones 
among the higher mammalia generally. By chafing their superior 
surfaces with the edge of the scalpel the grit of the ossificatory state 
can at once be detected, though they are yet thin and easily punct- 
ured. Each one is in contact, laterally, for its entire length with 
the premaxillary of its own side. In front they overlap the cartila- 
ginous snout, while posteriorly they are separated from the frontals 
by a membranous suture, the coronal suture of anthropotomy. They 
curve downwards mesially, and are in contact with each other for 
their entire lengths. 
Afrontal, as another investing membrane bone, seems to be rather 
further advanced towards ossification than we found a nasal to be, 
and this pair of bones are, as before remarked, the chief roofing ele- 
ments at this stage of the middle area of the skull. Behind and 
towards the middle line each one presents a rounded angle to the 
" fontanelle " (fig. 2, /o.), and is separated from the parietal of its 
own side by a membranous interval. Within either orbit a frontal 
is in contact with the nascent maxillary margin in front and below, 
while mesoposteriorly, and at the side, the bone is juxtaposed to the 
growing margins of the ali- and orbitosphenoids and the squamosal. 
(Plate I, Fig. 1). 
Mesially, the parietals are well separated from each other by the 
intervention of the " fontanelle " (Plate I, Fig. 2, fof. Here, they 
are at this stage antero-posteriorly rather narrow, but each one be- 
comes gradually broader as it proceeds outwards and curves down- 
wards towards the lateral aspect of the cranium, where the bone is 
separated by a considerable membranous sutural interval from the 
squamosal for its anterior marginal moiety ; its posterior border 
jutting freely into the unossified tract above and behind the auditory 
bulla of the same side. Either parietal shows even better evidence 
of ossification than the frontal in front of it, being best marked at the 
middle of the bone, while the periphery is yet largely membranous, 
which fact no doubt allows the bones to assume the position they 
eventually attain to in the skull of the adult Neotoma. 
There is a large and distinct interparietal, which the writer is 
inclined to think ossifies from two centres, one on either side of the 
middle line, though at the present stage it is one piece, and more 
thoroughly ossified than any of the elements described in the last 
few paragraphs (Plate II, Figs. 1, 2 and 4, i. pf. It has the 
PROCEEDINGS OF THE ACADEMY OF 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
17 
form of a lune, its straight edge being directed anteriorly, and the 
middle third of the same forming the hinder boundary for the 
"fontanelle," while its arc behind is juxtaposed to the non-united 
supraoccipitals, the extremities of the bone jutting at either lateral 
aspect into the unossified area above the auditory bulla. In an 
adult Neotoma cinerea the interparietal can be easily made out by 
its sutural margins, and it will be seen that it comes there more to 
contribute to the cranial roof, rather than to close in the back of the 
brain-case as it does in our embryo N. fusdpes. 
Of all the investing bones of the outer surface of the cranium in 
our specimen none possess a greater interest than the squamosal. 
One of these stands between the frontal and parietal of the same 
side above, and the primary structures of the ear and associate parts 
below. Essentially, at this stage the squamosal is a flat bone of a 
pear-shaped outline, the smaller part being directed backwards, and 
the larger oval moiety forwards, which latter bears near its anterior 
periphery the forward-projecting zygomatic process. It articulates 
with the posterior extremity of the jugal of the corresponding side. 
Even in the adult Neotoma, the glenoid facette of the squamosal is 
not nearly as definitely marked as we find it in many other forms of 
eutherian mammals, so in this embryo the feature is still less pro- 
nounced at the stage of development in the specimen before us. In 
figure 1 of my plates I have intentionally slightly dislodged the 
mandible from its normal position the better to show the embryonic 
otic structures, which latter have to a less degree been similarly 
dealt with. 
Apparently one of the scale-like, small, and fairly well ossified 
lacrymals presents a greater surface to the facial aspect of the skull 
than it does to the orbital, and at this stage the lacrymal canal 
(Plate I, Fig. 1, I. c.) may be seen just anterior to the slit-like and 
extensive infra-orbital foramen. 
Professor Flower has said that this orifice "is always well within 
the margin of the orbit,"1 but this wood rat undoubtedly constitutes 
an exception to the rule, for in the embryo it is found as I have just 
described it, and in the skulls of the adult specimens of N. cinerea 
at my hand, the lacrymal canal is found within that vertical fissure 
which in these rodents represents the infra-orbital foramen. It 
is here quite large, wedge-shaped, the edge being below, and its 
roof above formed by its maxillary, which base also forms the verti- 
1 Flower, W. H. Osteology of the Mammalia, 3d Edition, p. 181. 18 
PROCEEDINGS OF THE ACADEMY OF 
[1889. 
cal plate of its outer boundary, while its inner wall is formed chiefly 
by the lacrymal, and to a very small degree, posteriorly, by the 
frontal. The j ugal takes no part in its formation in Neotoma. 
Rodents are notorious for the large size of the premaxillary bones 
in their skulls, and to this rule the genus now under consideration 
forms no exception. Here a premaxillary of either side stands 
between the corresponding nasal and maxillary, preventing those 
elements of the face from coming in contact at any point. Supe- 
riorly, it sends backwards a lanceolate-shaped apophysis to overlap 
the frontal of the same side, and this feature is distinctly shown in 
Plate I, Figs. 1 and 2. 
Laterally, it makes up the chief side-wall of the conical fore-part 
of the skull of this embryo, and rounds under to form the roof of the 
anterior part of the mouth, though here a mesially oval foramen 
largely prevents it from suturally uniting with the fellow of the 
opposite side down the middle line (Plate I, Fig. 3). Each one 
anteriorly and beneath shows a diminutive pitlet, in which the inci- 
sor tooth is budding out, the latter not quite filling the hole closely, 
and withal being yet in an elementary state. Either premaxillary 
in the oval foramen above referred to, and on the outer aspect of the 
nasal septum in front, send backwards a teat-like process, into the 
spinal space; and in the skull of the adult Neotoma, these processes 
seem to have finally each pressed down upon the vomer and fused 
with its infero-anterior part. A premaxillary articulates with a 
nasal, a frontal, a lacrymal, the vomer, the ethmoidal mass, and with 
the fellow of the opposite side, and at this stage it seems to have 
progressed in its ossification, about as far as the nasals have, already 
described above. 
The maxillary, on either side, is a very important, not to say inter- 
esting, investing bone of the face and roof of the mouth (mx). It 
sends backwards a delicate zygomatic process, which underlaps the 
jugal to complete the zygoma. To some extent anteriorly, it con- 
tributes to the formation of the bony walls of the orbit, especially 
where it articulates with the lacrymal. In front, just beyond the 
infra-orbital foramen, it articulates by a vertical suture with the 
premaxillary of the same side, while behind this it sweeps down- 
wards and inwards, to complete the posterior moiety of its own side 
of the periphery of the incisor foramen, or " the anterior palatine 
foramen " of Flower, after which it contributes to the osseous oral 
roof, articulating by a transverse zigzag suture with the palatine, and 1889.] 
at the lateral aspect developing at this stage the elementary alveolar 
" process " for the lodgment of the molar teeth. These latter are 
now in a very primitive condition, though their form can be made 
out with no little distinctness. As in the case of the incisor teeth, 
their pristine sockets are larger than is necessary to lodge their 
dental occupants. 
According to Wiedersheim, the jugal (Plate I, Figs. 1,2 and 3J) 
is considered to be one of the investing bones of the outer side of the 
mouth cavity, as the vomer is considered an investing bone of the 
mouth cavity proper. In Neotoma, as in some other rodents, the jugal 
has an uncommon disposition, as it is overlapped by the zygomatic 
process of the squamosal, and underlapped by the maxillary. For 
the most part it is to be found lying along on the superior aspect of 
the maxillary, while but a limited portion of it is to be seen upon a 
better view of the arch, at the middle of its continuity. In neither 
the embryo N. fuscipes, nor in adult specimens of the genus, does 
the jugal possess any salient characteristics, its sole function being to 
complete the slender zygomatic arch, and were it not that the de- 
mands of a universal law exacted its presence, its actual use might 
easily be dispensed with, for the zygomatic arch could just as well be 
completed by a meeting of the maxillary and zygomatic process. 
Indeed in some adult skulls of N. cinerea, it takes a good lens and 
careful observation to detect the presence of the jugal bone at all, so 
perfectly is it moulded to the conformation of this delicate osseous 
rod, which curves below and well out beyond the orbital cavity, in 
one way defining its limits, and lending to the skull of this species 
its well-known and characteristic form. 
The vomer presents nothing peculiar, contributing as it does to the 
hinder moiety of the nasal septum, and ossifying rather late in the 
growth of the skull. We may refer to this bone further on, when we 
come to investigate the formation of the rhinal chambers. 
We next pass to a brief consideration of some of the most interest- 
ing structures in this or any other mammalian skull; I refer to the 
mandible and those parts at its posterior extremity which are con- 
cerned in the elaboration of the auditory apparatus. Here, to some 
extent, we are upon debatable premises, and are brought face to face 
with yet mooted questions in morphology, and consequently must 
proceed with caution. Plate I, Fig. 1 shows very well the form of 
the mandible upon its lateral aspect. At this stage it is deep and 
thick, although the symphysis anteriorly has not as yet coossified, the 
NATURAL SCIENCES OF PHILADELPHIA. 
19 20 
PROCEEDINGS OF THE ACADEMY OF 
[1889. 
two halves here being held in juxtaposition by the not inextensive 
union between the fore ends of the Meckelian cartilaginous rod. 
Those toothsacs for the lower incisor teeth are about as far advanced 
as those described for the premaxillary bones, and this applies with 
equal truth to the molars of this lower jaw, though perhaps, these are 
not quite as far advanced as the molars of the maxillary above. 
For the most part the mandible has to no small degree ossified. 
We are to note, however, that the summit of the low and ill-defined 
"coronoid process" is tipped with nascent cartilage, and this state 
obtains also with the yet growing condyle, here harbored in the shal- 
low glenoid cavity of the squamosal on either side. Below these 
protuberances a prominent angular process is to be observed, also 
cartilage-tipped as in the case of the condyle and coronoid. These 
several prominences of the posterior end of either ramus of the lower 
j aw in this embryo rodent, gives it a very marked vertical depth and 
a fan-like form, which renders it quite conspicuous. Through the 
ramus and thoroughly ensheathed by it, longitudinally courses the 
slender Meckelian rod of cartilage, to fuse anteriorly, as already 
stated, with its fellow of the opposite side. 
At its hinder and free end, the Meckelian cartilage becomes clubbed 
and of a peculiar form; the extremity proper is cupped to receive 
in articulation the incus, while below this cup the rod sends forwards 
and downwards a very delicate, cylindrical spur of cartilage, here 
incorporated in the plane of the membrana tympani, within the semi- 
arc of the tympanic annulus. These parts seem yet to be largely 
performed in cartilage, and we see the malleus in the hinder end of 
this Meckelian cartilage, while the labors of Salensky, Fraser and 
Kitchen Parker seem to have at last definitely decided that " the 
incus is the upper element of the first or mandibular arch." 
Here in this embryo Neotoma, the incus develops an unusually long 
posterior crus, as shown in Figure 1, extending over towards the 
auditory bulla (an). As in the vast majority of the higher groups 
of the mammalia, the stapes is stirrup-shaped, and its foot-piece 
closes up the fenestra ovalis, and this stapes according to the most 
recent researches has been said to correspond with the hyomandibu- 
lar of fishes, or in other words is the upper element of the hyoid arch, 
as the incus, as we have just said, is the off-constricted piece of the 
proximal extremity of the mandibular arch. Agreeing with the 
first two described auditory ossicles, the stapes at this stage in Neo- 
toma fuscipes, seems yet to be wholly in cartilage. I failed to detect 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
21 
the rudimentary interhyal in the tendon of the stapedius muscle, but 
might do so were additional material at hand. These wood rats have 
tympanic bullae of no inconsiderable size, and in the dried skull of 
an old N. cinerea their lower surfaces are quite transparent, each 
being somewhat laterally compressed and inclined towards each 
other, so that were their imaginary horizontal long axes produced 
to the front they would intersect at a point just slightly anterior to 
the posterior narial aperture. 
In form, then, these bullae are subcompressed ovoid, where mesi- 
ally they are moulded upon the bones they come in contact with 
at the base of the cranium at its infero-external aspect, which have 
to do with the auditory chamber. 
Turning for the moment from our consideration of the proper 
" investing bones," I desire to pass a few remarks upon the " hyoi- 
dean apparatus." Essentially, this seems to be built up upon the 
plan of these parts as we find them in the more highly organized 
eutherian mammalia generally. In the specimen before us, however, 
ossification even at this stage appears but to have advanced slowly, 
the several segments of the arch yet being largely in cartilage. 
All the elements of these parts seem to be present in this embryo, 
and the most notable feature to me is the form of the basi-branchial 
as I make it out. It is far more extensively curved than Parker 
found it to be in the hedge-hog, by which I mean the curved rod of 
cartilage composing it is longer (Plate I, b. h. br.~). By examining 
the arch in an adult N. cinerea, the same feature seems to be present, 
for the "body of the hyoid" is there'larger and curved. Another 
mammal, the dog, as drawn by Flower, also shows a curved basi- 
branchial, whereas in the armadillo it is a medium piece united 
apparently with the thyrohyals (Parker). The thyrohyals in the 
embryo Neotoma are but feebly developed (t hy), and remain in- 
conspicuous after ossification in them is completed. Agreeing with 
most ordinary mammals the three remaining pieces of the hyoid 
present nothing of marked peculiarity. 
Returning to the investing bones of the skull, there yet remains 
two of them to be described in the present connection, viz.:-the 
palatine and the pterygoid. 
If there be any superficial ossific deposit yet in either pterygoid 
(Plate I, Fig. 3, pg) the writer failed to discover it after a very 
careful investigation. We are aware that in the pig at the fourth 
stage of its development the pterygoids are somewhat advanced in 22 
PROCEEDINGS OF THE ACADEMY OF 
[1889. 
their ossification, but here in our specimen each pterygoid proper 
now consists in a well-pronounced subcylindrical cartilaginous rod- 
let to some extent centrally ossified, which, on either hand, projects 
out from behind the ossifying palatine of its own side. External 
to either one of these we observe the continuation of the bone in its 
' external pterygoid plate,' which is also in cartilage, and arises 
both from the ali- and basisphenoidal regions. Neotoma cinerea, in 
the skull of the adult, shows well how the pterygoids are finally 
fashioned in bone at maturity, and if they represent the condition 
for the genus, we find that the external plate in each is nearly hori- 
zontal in position, and raised above the frontal and sphenoidal 
regions; the sutural traces remain quite distinct; the " hamular 
processes " have their extremities produced, and turned slightly 
outwards, while these apophyses are not far apart mesially, nor do 
their ends lack much of reaching the auditory bulla on either side, 
where the Eustachian tube opens. 
Passing next to a palatine bone, we find it to be ossified to no in- 
considerable extent, with the salient angles of its horizontal portion 
rounded off, its borders being yet membranous, or in an imperfect 
cartilaginous stroma, a tissue furnished by the epiblast during the 
growth of the embryo, and now taking on ossification. 
The G posterior palatine foramina " are easily found, while the 
hinder margins of these bones unite to form the lower free edge of 
the posterior bases. 
Having the most usual relations to the nasal septum and surround- 
ing parts, the ascending lamina of a palatine is not so thoroughly 
ossified as the horizontal portion of the bone ; and at this stage of 
its growth, this embryo Neotoma may be said to have a cleft palate, 
so wide are the medial sutures between the elements. 
To Professor Kitchen Parker are we indebted more than to any 
other single writer since the dawn of anatomical science to the pres- 
ent time for our knowledge of the morphology of the vertebrate 
skull, and from his recent writings, the mammalian skull in partic- 
ular. Under the influence of his never weary hand, has this, one of 
the most difficult problems which man has ever investigated, grown 
out into the light with all the beauty associated with the budding of 
a complete flower. In reading his " Structure and Development 
of the Skull in the Mammalia" one loses himself as though he were 
Of the endocranium, and the development of the cartilage bones. 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
23 
perusing pages of the most fascinating romance, and is led on step by 
step, as " the thing" grows, and shapes, and matures, with an almost 
irresistible passion for the marvelous story. Omitting those parts 
which I have referred to above, and confining ourselves to the 
structures which properly fall within the present section, we are 
clearly shown how in the endocranium in its pristine membranous 
condition there is laid down at its base in primitive cartilage the 
pair of rods which are the ground plan of the future brain-case, the 
harborage of that most powerfid of all organs, the encephala. These 
rods of the trabeculae cranii, which behind embrace the notochord 
(the parachordals), while anteriorly their segmentations become the 
trabeculae proper and enclose the primitive pituitary space. 
From these simple beginnings, part after part, grows and evolves, 
until in due time we have before us the mature skull with all its 
associated structures. At the appointed instant sense-capsules are 
born and elaborated pari passu as the cranial moulding proceeds; 
and nerves and vessels burrow' with precision through tracts and by- 
ways long known to their kind in the ancestral types of the species, 
guided by the ceaselessly acting laws of variation and evolution. 
Turning to the nether aspect of the basis cranii in our embryo Neo- 
toma fuscipes we find the foramen magnum (Plate I, Fig. 3,f. mi) to 
be of a subelliptical outline, a form retained probably throughout 
life, as it obtains in the skulls of other adult Neotomas which I have 
examined. The supraoccipital is still in two parts, the medial ver- 
tical suture being very evident. It is, however, rapidly ossifying, 
bone having advanced to the superior arc of the foramen, and no 
doubt that early in the next stage the supraoccipital would be in 
one piece (s. o.). 
The condyles show very prettily, and as structures developed by 
the exoccipitals (e. o.) they are well started in the process of ossifica- 
tion, though their ossific centres have not yet impinged upon either 
the supra or basioccipital. Much cartilage is still to be found, both 
above them and at their sides. 
Embedded in this material below, we are to observe just in front 
of the occipital foramen the subquadrilateral form of the basiocci- 
pital, already nicely started in bone. This osseous part does not as 
yet reach the auditory bulla on either hand, though posteriorly it 
arrives at the margin of the foramen magnum. Anteriorly, a car- 
tilaginous tract intervenes between the concave border of the basi- 
occipital and the ossific centre which represents the future basisphe- 24 
PROCEEDINGS OF THE ACADEMY OF 
[1889 
noid. This latter (6s) is at present but a small, squarish piece of 
bone occurring in the basic cartilage, just posterior to the pterygoid, 
which in the skull of an old N. cinerea, is wedge-shaped in outline 
for its exposed surface, and stands between the pterygoids, the nar- 
row end to the front, separated by a distinct suture from the pre- 
sphenoid, and the broad end behind, separated by a similar suture 
from the basi-occipital; the three bones, thus continuous, making a 
characteristic area having the form of an acute isosceles triangle. 
Nearly every trace of the notochord has disappeared in our embryo 
Neotoma at this stage, its former presence being but faintly indicated 
by a whitish line traversing the basi-occipital plate in a medio- 
longitudinal direction, and entirely disappearing near its middle. 
Beyond, the presphenoid shows commencing osseous deposit in a 
narrow line down its length, but is still chiefly performed in cartil- 
age, the former being barely perceptible. Referring again to skulls 
of the N. cinerea, adult specimens, it becomes worthy of remark, that 
the supraoccipital region and the foramen magnum are both in near- 
ly the same plane, it being quite vertical, and almost at right 
angles with the horizontal plane in which the interparietal and 
parietals lie. This part of the cranium in our embryo, as already 
stated above, is more or less rounded as shown in the figures. To 
the outer side of either exoccipital is seen a distinct and spine-like 
paroccipital, which feature I fail to find in so early a stage as the 
embryo before us represents. 
Professor Parker in his famous work upon the ' Morphology of 
the Skull ' in alluding to the development of the pig at its ' fourth 
stage,' contends that there the notochord is not yet quite obliterated 
in the basioccipital, though it is rapidly becoming so. He also 
points out that a separate ossific centre, in that animal, is to be 
found in either massive condyle, but they soon coalesce with the ex- 
occipitals on either side. Agreeing with Neotoma fuscipes, the supra- 
occipital in the pig at this stage is in two pieces, or " patches " as 
Professor Parker expresses it, and they " run into one another in a 
day or two " (p. 288). 
Of the three bones that unite to form the periotic ossification, I 
find but one that as yet appears in any way advanced beyond a car- 
tilaginous condition ; and this is the opisthotic (Plate II, Fig. 5, o.p.~) 
After the membrane bones which form the vault of the skull have 
been duly removed, this auditory osseous element may be detected 
posterior to, and to the other side of the periotic capsule, which is 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
25 
otherwise in nascent cartilage, though quite dense and on the verge 
of ossifying. This internal view of the cranial casket, reveals the 
fact that at this stage at least the periotic mass is but slightly eleva- 
ted above the general level of the floor of the brain-case, and in no 
way as prominent as the external auditory bulla on either side. It 
still, however, is more or less rounded, and it is only later in life 
apparently that this projection becomes somewhat angulated as we 
see it in the adult skull. Tn this region in the cranium of the em- 
bryo Neotoma the usual vascular and nerve foramina can be easily 
made out. 
Already in another paragraph, I have alluded to the state in which 
we find the basi- and presphenoid (Plate II, Fig. 6, b. s. and p. s.) 
and it will be seen that the anterior part of the cranial floor is fairly 
well-paved by two other ossifications of mammalian skull. I refer 
to the rather large and squarish alisphenoids (al. s.), one on either 
side of the first-named element, and the more oval orbitosphenoids, 
one on either side of the presphenoidal cartilage. These bones are 
already well-ossified and are pierced by the usual nerve foramina ; 
the latter by the optic (II), and the former by the third branch of 
the fifth (V3) through the foramen ovale. A slight pituitary depres- 
sion is to be observed at its most usual site upon the presphenoid. 
The arrangement of these parts then, with its true stirrup-shaped 
stapes, indicates that Neotoma holds quite an exalted position, struct- 
urally, among the eutherian mammalia. 
Beyond the sphenoidal region the endocranium gradually but 
rapidly narrows, and just posterior to the nasal structures and cham- 
bers we meet with a sub-vertical cribriform plate (er. p.') showing mi- 
nute perforations for the passage of the nerves. Mesially, a posterior- 
ly rounded crista galli (Plate II, Fig. 5, er. y.~) is to be seen. These 
parts are as yet all performed in cartilage. 
By carefully removing the investing bones from the fore part of 
the skull we at once bring into view the chondrified elements of 
the nasal organs. We have already alluded to the vomer, and now 
it can be easily detached from the cartilaginous lamina perpendicu- 
laris, with which it is articulated by a longitudinal median groove 
traversing its entire superior margin. 
The base, for the vomer at this stage is entirely in bone, is some 
3 cms. long, and sharp, or more or less sharp, along its inferior edge. 
As in most rodents the lamina perpendicularis is produced forwards 26 
PROCEEDINGS OF THE ACADEMY OF 
[1889. 
as the septum nasi (Fig. 6, s. n.). Cribriform plate and lamina per- 
pendicularis together form the mesethmoid. 
Jacobson's Organs, one on either side of the septum, are, as in the 
vast majority of the rodentia, well developed in this embryo; and 
as in the case of Lepus appear to be ensheathed by the backward, 
extending, posteriorly free terminating palatine processes of the 
snout, as seen in Figure 6 (re. c.). These " organs " have been 
described by Prof. Wiedersheim (Parker's translation) as being " a 
paired accessory nasal cavity, which in an early embryonic stage be- 
comes entirely separated from the nasal chamber, and which is 
supplied by the olfactory and trigeminal nerves." They are sur- 
rounded by the cartilages of Jacobson, likewise paired and of a scroll- 
like form. Uniting with the antero-ventral aspect of the septum 
nasi beneath, we are to note the ali-nasal cartilage on either hand, 
encircling tlie external nostrils (e. n. and al. n.~), while the 
one on each side of the septum on the dorsal aspect, run the entire 
length and roof over the nasal chambers (Plate II, Fig. 5, al. sp.f 
Large and of peculiar form, the aliethmoids form a striking feature 
here, and may be seen both upon dorsal and ventral aspects. To 
some extent they form the roof of each rhinal chamber, as well as 
the sides and floor.- Inferiorly, they become much expanded behind, 
rounded, encircling submesial vacuities, as shown in Figure 6, (Plate 
II, al. e.). 
Finally, turning to the ventral aspect of this minute endocranium 
we see external to either recurrent cartilage, right and left of the 
median ossified vomer (v), the inferior turbinal proper (i. tb.~), each 
one in addition to its usual attachments in mammals of this order 
is here connected with the septum nasi anteriorly, and terminates 
behind in a teat-like process, extending somewhat further in 
that direction than does the vomer. They swell at the middle of 
their continuity, being convex externally and concave upon their 
vomerine aspects. In a former paragraph, I have already alluded 
to the " recurrent cartilages," so designated by Parker (re. c), and 
these may now be seen upon this view. Especial attention is invited 
to them as they are the essential capsules which enclose the organs 
of Jacobson. To study them properly it becomes necessary to gently 
press outwards the inferior turbinals. At present they are in car- 
tilage entirely, thin, and of an elegant scroll-like form, being for the 
most part open superiorly, and in contact with the vomer, mesially, 
being in either case outgrowths of the alee nasi. 1889.] 
NATURAL SCIENCES OF PHILADELPHIA. 
27 
With this I close the present account of the cranium in this em- 
bryo of one of our not abundant species of American rodents. Upon 
some future occasion I trust to compare it quite extensively with the 
development and morphology of the skull in other types of mammals 
of this country, especially the rodentia. 
Explanation of Plates I and II. 
Note.-The figures of these two plates were drawn and colored 
by the author directly from his dissections, ami in them the mem- 
brane bones have been simply shaded; the cartilage bones colored 
orange ; and the endocranium and other cartilaginous parts, purple. 
In all the figures the structures are increased six times the size of 
life, and the following letters used as abbreviations: 
ay. p. Angular process. 
al. e. Aliethmoid. 
al. n. Alinasal. 
al. s. Alisphenoid. 
al. sp. Aliseptal. 
a. ty. Annulus tympanicus. 
au. Auditory capsule. 
b. h. br. Basihyobranchial. 
b. o. Basioccipital. 
b. s. Basisphenoid. 
cr. y. Crista galli. 
er. p. Coronoid process. 
er. pl. Cribriform plate. 
d. Dentary. 
e. hy. Epihyal. 
e. n. External nostrils. 
/. Frontal. 
f. m. Foramen magnum. 
fo. Fontanelle. 
gl.f. Glenoid cavity. 
i. Incus. 
i. p. Interparietal. 
i. tb. Inferior turbinal. 
j. Jugal. 
I. Lacrymal. 
I. c. Lacrymal canal. 
mk. Meckel's cartilage. 28 
PROCEEDINGS OF THE ACADEMY OF 
[1889. 
ml. Malleus. 
m. ty. Membrana tympani. 
mx. Maxillary. 
n. Nasal. 
nf. Nasal floor. 
n. v. Nasial valve. 
oc. c. Occipital condyle. 
op. Opisthotic. 
o. 8. Orbitosphenoid. 
p. Parietal. 
pa. Palatine. 
p. e. Perpendicular ethmoid. 
pg. Pterygoid. 
ps. Presphenoid. 
pm. Premaxillary. 
py. Pituitary region. 
re. c. Recurrent cartilage. 
s. n. Septum nasi. 
s. o. Supraoccipital. 
sq. Squamosal. 
st. Stapes. 
t. hy. Thyrohyal. 
v. Vomer. 
Plate I. 
Fig. 1. Left lateral view of the skull of an embryo Neotoma 
fuscipes, with hyoidean apparatus attached ; X 6. 
Fig. 2. The same specimen seen from above; mandible removed. 
Fig. 3. The same from below; mandible removed. Roman 
numerals indicate nerves or their foramina. 
Plate II. 
Fig. 4. Posterior view of the skull of an embryo Veoioma fusci- 
pes; same specimen as figured in Plate I, (X 6) ; mandi- 
ble removed. 
Fig. 5. The endocranium of a specimen of an embryo of Neoto- 
mafuscipes; the same skull as is figured in Plate I, fig. 
1 (X 6). All the investing bones have been removed, 
and superfluous parts dissected away. 
Fig. 6. The same specimen shown in Fig. 5, under view, Proc.Acad.Nat.Sci.Phila. 1889 . 
Plate I . 
ng.i. 
Kg 2. 
Kg-3 
R. .W. Shufeldt ad. .nat. del. 
Geo-.S Harris & Sone Lith.Phila. 
SHUFELDT ON NEOTOMA FUSCIPES. 
(Embryo x6). Proc.Acad.Nat.Sci.Phila. 1889 . 
Plate II . 
Fig.4. 
Fig.5. 
R W Shufeldt ad.nat.del 
Geo.S . Harris & Sone Lith.Phila. 
SHUFELDT ON NEOTOMA FUSCIPES. 
(Embryo x6).