TRACHYCARCINUS. 25 
Famity CORYSTID &. 
TRACHYCARCINUS Fax. 
Bull. Mus. Comp. Zool., XXIV. 156, 1893. 
Carapace pentagonal, moderately convex, lateral margins long, nearly 
straight, toothed. Front narrow, produced, three-toothed. Orbits large, 
with forward aspect, imperfect, with two hiatuses above, one below, and one 
at the inner angle; lower wall formed chiefly by the carapace. Anterior 
margin of buccal cavity not distinctly defined, epistome short, ridges of the 
endostome developed. Sternum long and rather narrow. Abdomen of male 
narrow and five-jointed, the third, fourth, and fifth segments consolidated, 
Eye-stalks very small, retractile within the orbits. Antennules longitudinally 
folded. The antenne lie in the inner hiatus of the orbit ; their basal segment 
is but slightly enlarged, not filling the hiatus at the inner angle of the orbit 
nor attaining to the front, subcylindrical, unarmed, imperfectly fused with 
the carapace ; the second segment is longer and slenderer than the first, the 
third segment about equal to the second in length, but slenderer ; all these 
segments are furnished with long and coarse sete; the whole antenna is 
less than one half as long as the carapace. The ischium of the outer maxil- 
lipeds is produced at its antero-internal angle; the merus of the same 
appendages is rounded at the antero-external angle, obliquely truncated 
but not emarginated at the antero-internal angle, where it articulates with 
the following segment. Legs of moderate length. Right and left chelipeds 
very unequally developed in the male. Dactyli of ambulatory legs styliform, 
straight, slender, longer than the penultimate segments. 
The pentagonal shape of the carapace recalls the genus Zelmessus White. 
But in Zelmessus the front is divided by a median notch, the orbit is much 
more complete, the basal segment of the antenna sending off an external 
process that completely fills the hiatus at the inner angle of the orbit. 
Although Zrachycarcinus bears but little resemblance superficially to Fricho- 
pelarion A. M. Edw.* on account of the very different shape of the carapace, 
it is in reality closely related to the latter genus as is shown by the close 
resemblance between them as regards the orbits and antennx, the merus 
of the outer maxillipeds, the form of the chelipeds, ete. 
The Corystoid crabs form a heterogeneous assemblage of rather primi- 
* Bull. Mus. Comp. Zool., VIII. 19, Plate IT., 1880. 
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