60 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



of them, which are probably connected with more important dif- 

 ference of the internal parts. One of these types is indicated in 

 the two strongly plicated species of the Oriskany sandstone, which 

 have a median sinus near the front of the ventral valve, with two 

 of the plications often closely incurved. Another type is that of 

 Leptocoslia concava and allied forms, which are more finely plicated, 

 and having a sinus on the dorsal valve, though not distinctly 

 defined. The third type is represented in Leptoccelia {Mtrypa) 

 planoconvexaj which has a somewhat undefined depression on the 

 dorsal valve, and a form of cardinal process unlike the other 

 species.* The internal structure of this species is still unknown. 



The Leptoccelia imbricata proves to be a Trematospira ; and 

 the same characters are apparent in L. disparilis of the Niagara 

 group, the concavo-convex form of the shell being the only appa- 

 rent deviation from typical forms of that genus. The Terebratula 

 lepida of Goldfuss, as shown in the collections of Dr. E-ominger, 

 possesses internal spires precisely similar to those of Trematospira 

 camura. 



The Leptoccelia concava, both in its external characters and in 

 the arrangement of the crura and vinculum, difi'ers from Trema- 

 tospira; and with the knowledge at present possessed, I am 

 compelled to separate this species from those last named, and 

 from the L.Jlabellites, L. fimbriata and L. acutiplicata. I would 

 propose to indicate forms of this external character, with similar 

 crura and spires, as Ccelospira. 



CCELOSPIRA CONCAVA. 



The difficulty constantly attending the references of the Bra- 

 CHiopoDA to established genera from external form and characters, 

 renders it very desirable to search- for the interior organization 

 and appendages; but the condition of specimens does not always 

 admit of satisfactory investigations, and not unfrequently the 



* Neither in this species, nor in the Retzia ovalis which has a similar forrii, have I 

 been able to determine the existence of a punctate structure of the shell. The latter 

 has internal spires, a feature not yet obserod in L. convex a. 



