DESCRIPTIONS OF TWO NEW SPECIES OF CRABS FROM 
THE WESTERN INDIAN OCEAN, PRESENTED TO THE 
NATIONAL MUSEUM BY DR. W. L. ABBOTT. 
By Mary J. Rathbun, 
Aid, Department of Marine Invertebrates. 
A large number of crabs were recently collected by Dr. W. L. 
Abbott at the islands north of Madagascar. Among them are two new 
species, both of which represent rare and peculiar genera. The genus 
Hypoccelus of the Caucrhhe can be distinguished by the oblong or oval 
cavity beneath the antero lateral border of the carapace. Three species 
have already been described: 11. granulatus, (De Haan), from Japan, 
H. diverticulatus, (Strahl) \=Caneer sculptus, Milne-Edwards, not 
Herbst], found sparingly from Japan to the Red Sea and Mauritius, 
and H. punctatus, Miers, of which a single specimen is known, from 
Torres Straits. 
The other new form in the Abbott collection is a member of the Tliel. 
plmsan genus Deckenia, the type of which was described by Hilgendorf 
from the adjacent African continent. This genus differs from other 
Thelphusidse in having the efferent branchial channel prolonged to the 
front, a character in which it approaches the Oxystomata. 
Carapace shaped much asiu H. punctatus * and strongly lobulated as 
in that species; but the second lobule near the antero lateral margin is 
longer than wide, and the posterior margin of the cardiac region is 
transverse and is distinctly separated from a small median lobule. The 
surface is rough with blunt spiniform tubercles, which also border the 
somewhat truncate frontal lobes and the prominent praeorbital pro- 
tuberances. There is a spine at the inner suborbital angle. The 
pterygostomian cavity is suboval, wider at its inner than its outer end. 
The anterior margin is straight for nearly its whole length and is 
formed by the antero lateral margin of the carapace. The cavity is 
crossed by two ridges nearly parallel to the anterior margin, the ante- 
HYPOCCELTJS ABBOTTI, new species. 
* Miers, Crustacea H. M. S. Alert, p. 206, pi. xix, fig. B, 1884. 
Proceedings National Museum, Vol. X VTII—No. 979. 
21 22 
CRABS FROM THE INDIAN OCEAN—M. J. RATH BUN. 
VOL. XVII. 
rior ridge stopping short of the inner margin, the posterior ridge shorter 
and not reaching the outer margin. The surface of the abdomen and 
sternum is covered with bead-like tubercles except for irregular eroded 
channels or pits. The right cheliped (the left one is missing) is massive 
and tuberculous or spinous. The merus is short and thick, margins 
tuberculous, inner and outer surfaces smooth, lower surface linely tuber- 
culous. Carpus with lower half of outer surface spinous. Manus with 
upper surface subtriangular, half as broad as long; outer surface with 
longitudinal spinous ridges, two of which are continued on the pollex 
and terminate in two of the live strong teeth (one is terminal) of the 
prehensile edge. The dactylus bears four spinous ridges on its outer 
and upper surface and seven teeth on the prehensile edge which are 
smaller than the propodal teeth and tit closely into the spaces between 
them. The inner surface of the maims is tuberculous on its proximal 
lower portion. The ambulatory legs are short and broad; the last two 
pairs when drawn up, lit into the cavity adjoining the postero lateral 
border of the carapace. The meral joints are very broad (in the first 
pair the width is one-half the length), and are hollowed beneath to 
receive the two following joints; their anterior distal angles are pro- 
duced and rounded. The distal end of the propodus is much broader 
than the adjacent portion of the rather slender and slightly tapering 
dactylus. 
Unfortunately it is impossible to give an accurate description of the 
natural color of the specimen, as with other crabs it was placed when 
collected in contact with nudibranch mollusks, which have given it a 
dark blackish-blue color. The entire surface is coated with a mem- 
branous covering apparently epidermic which tends to obscure the 
tubercles. Where this has been removed with muriatic acid the cara- 
pace appears to be blotched with bright red and white. The tips of 
the spines are white. The carapace is sparingly hairy, the legs are 
margined with hair, and the subbranchial regions are very hairy. 
Measurements.—Length of carapace (of male), 55.5 mm.; width, 7G; 
length of manus, lower margin, 44; depth, 21; width of upper surface, 
10; length of merus of last ambulatory leg, upper margin, 10.5; length 
of carpal joint, 11; length of propodal joint, 0; length of dactylus, 10; 
width of merus, 0.5; proximal width of propodal joint, 0; distal 
width, 4; greatest or transverse diameter of pterygostomian cavity, 
25; opposite dimension, 11. 
Locality.—Aldabra Island. (One male, No. 17753). 
A. Milne-Edwards in describing the genus* says that the pterygo- 
stomian cavity is concealed by the anterior feet when the latter are 
folded against the carapace. This is not the case in this species. 
In no position does the cheliped fit tightly over the cavity. When the 
cheliped is folded, the upper surface of the manus is continuous with 
* Nouv. Arch. Mus. Hist. Nat, Paris, i, 1865, p. 295. 1894. 
PROCEEDINGS OF THE NATIONAL MUSEUM. 
23 
the inner distal portion of the outer face of the carpus. The inner 
border of this area corresponds in position very nearly with the posterior 
margin of the pterygostomian cavity; the cavity itself is therefore left 
uncovered. The only other representative of this genus in the National 
Museum is a single specimen of H. diverticulatus, from Mauritius, in 
which the cavity is longitudinal, its margin formed by two confluent 
circles. When the cheliped is folded against the body, the concave 
upper margin of the manus and anterior carpus crosses the cavity at 
its middle, concealing the posterior half, and exposing the anterior 
half. 
II. abbotti differs from other species in the double ridge in the ptery- 
gostomian cavity, and in the carapace marked with tubercles instead 
of granulations, rugose lines, or punctsc. 
Carapace a little more than four-fifths as long as broad, very thick, 
slightly convex transversely, very convex longitudinally, antero-lateral 
margins strongly arcuate. There is a prominent tuberculous post- 
frontal crest, extending across the carapace, sinuous, interrupted at 
the median line and at the cervical suture, and slightly interrupted 
behind the base of the eye. The median groove extends backward 
from the frontal margin to the postfrontal crest, where it divides into 
two grooves outlining the narrow anterior portion of the mesogastric 
region. The cardiacal grooves are shallow; the cervical groove is 
almost longitudinal for a portion of its length, then curves outward 
and ends at the post-frontal crest. The dorsal surface is punctate, the 
anterior half is covered with squamiform granules, which, on the ante- 
rior branchial regions, become tuberculous and tend to form short 
ridges. The front is about one-third the width of the carapace, detlexed, 
tuberculous, three lobed, median lobe more advanced and much nar- 
rower than the lateral, its margin bent towards the horizontal. The 
margin of the front and orbit is raised, shining and indistinctly granu- 
lous. The orbital margin is sinuous, its general direction being outward 
and forward. The postorbital tooth is acute and more advanced than 
in I). imitatrix. The epibranchial tooth is smaller and not far behind 
the postorbital; it is followed by a row of about twenty small tubercu- 
lous teeth, forming an antero-lateral marginal line which posteriorly 
curves upward and inward on the carapace. The postero-lateral 
branchial regions are marked by transverse broken raised lines, which 
are continued on the subbranchial area. The posterior of these lines is 
the strongest and is continued further inward on the dorsal surface. 
The suborbital margin is nearly straight, inclined inward and slightly 
forward, and is finely toothed, its inner angle thickened and adjoining 
the end of the efferent branchial channel, which is in advance of the 
lateral frontal lobe and is visible from above. On the subhepatic region 
are two tuberculous lines concentric to the orbit. The abdomen of the 
DECKENIA CRISTATA, new species. 24 
CRABS FROM THE INDIAN OCEAN—M. J. RATTIBVN. 
female lias the first two segments short, the third to the sixth increasing 
successively in length, the terminal segment large, subtriangular, one- 
half as long as wide. Eggs very large, 34 mm. in diameter. The disposi- 
tion of the antennal region is similar to that of I). imitatrix. The anten- 
nae are minute, smaller than in that species, and smaller than the anten- 
nuhe. Maxillipeds similar in shape to those of 7). imitatrix; margin 
of nierus and anterior margin of ischium tuberculous. 
The clielipeds are very unequal and rough with transverse squamose 
lines. The lower and inner margins of ischium and merus are armed 
with triangular spines; the upper margin of the merus, with teeth which 
are prolongations of the rugosities of the outer surface. The carpus 
has a stout inner spine, with a smaller one at its base. The larger 
hand is deep and swollen; both hands are curved inward and very rough, 
especially above. Digits with impressed longitudinal lines; very stout 
in the large cheliped; prehensile edges irregularly toothed and almost 
touching. The ambulatory legs are longer than in T>. imitatrix. Joints 
margined, and with transverse rugose lines which form shallow teeth 
at the upper margin. Carpal and propodal joints with a double margin 
above, and two more or less rough longitudinal ridges on the outer sur- 
face. Propodal joints with two rows of small appressed spines below. 
Dactyli flattened, longer and narrower than in 7). imitatrix, with two 
rows of sharp spines above and below. 
Measurements.—Length of carapace (ot female), 29 mm.; width, 34; 
thickness, 18; width of front, 12; width between postorbital spines, 
22.5; width between epibranchial spines, 27; length of propodal joint 
of cheliped, lower margin, 27; depth, 13.5; thickness, 8.8. 
Locality.—Seychelles. (Two females, one with eggs; one young, ISTo. 
180G4). 
This species differs from Declcenia imitatrix from Zanzibar in the post- 
frontal crest, narrower front, rough surface, fewer spines on the carpus, 
and in so many other respects that the species are not likely to be con- 
founded. 
VOL. XVII.