148 S. I. Smith on American Crustacea. 



Tliere are two rather badly preserved specimens, collected at Paita, 

 Peru, by Prof. Jaraes Orton, in the Museum of Yale College. The 

 smaller specimen wants both chelipeds, and the larger specimen, one. 



This species is closely allied to _P. macropa, but is easily distin- 

 guished from it by the denticulated antero-lateral margin, by the short 

 merus of the chelipeds, and by the flattened carapax — the carapax of 

 _P. macropa being represented in Edwards' figure as quite convex 

 transversely, while in P. plana it is flat in that direction. Moreover 

 the front seems to be much more deflexed in our species, the orbits 

 are much smaller and are well filled by the eyes, and the antero-lateral 

 margin is not " creuses en dessous d'un sillon bien marque." In the 

 depressed form of the carapax, it is apparently closely allied to JP. 

 gracilijyes, but the ambulatory legs are not longer in proportion than 

 in jP. macropa., and the front is almost straight, as seen from above, 

 and not lobed as in P. Americana, with which the tront of P. gra- 

 cilipes is compared. In the denticulated antero-lateral margin it re- 

 sembles P. C'hilensis, but in the form of the carapax, and in other 

 characters it is much nearer to P. macropa. 



Opisthocera,* gen. nov. 



The carapax is much as in PseudothelpJuisa ; the dorsal surface is 

 not distinctly areolated ; the front is deflexed, smooth and unarmed, 

 and the edge is not reflexed beneath a superior crest as in Epilohocera 

 and Potamocarcinus ; and the lateral margins are not armed with 

 strong teeth or spines. 



The epistome is deeply channeled transversely and the labial bor- 

 der is divided into three very prominent lobes projecting far forward, 

 and of which the lateral ones are bilobed at tip and are separated 

 from the antero-lateral angles of the buccal opening by broad and very 

 deep efierent orifices. 



The external maxillipeds are as in Epilohocera, the merus trans- 

 verse, the anterior margin rounded, and the palpus goniarthroid. 



In the single species upon which the genus is based, there is a long 

 and slender spine projecting from the upper side of the expiratory 

 canal near the external orifice. 



In the character of the front, this genus agrees with the species of 

 Pseudothelphusa which have no superior frontal crest and diflers from 

 Epilohocera, while, in the position of the antennse, it agrees with Ejn- 

 lohocera and differs from Pseudothelphusa. 



* 'OtvcgOe, po7ie ; Kcpag, cornu. 



