PELEGYPODA. 205 



MOLLUSCA 



Class PELECYPODA 



ORDER PRIONODESMACEA 



Superfamily Pteriacea 



Family Pteriidae 



Genus Pteronitella Billings 



Pteronitella sp. 



Plate 54, figure 5 



Description — An internal cast of a portion of a left valve — with extended posterior 

 extremity, and marked at least with concentric lines and wrinkles — is doubtfully as- 

 signed to this genus. 



Family Conocardiidae 



Genus Conocardium Bronn 



Conocardiutn sp. 



Plate 54, figure 6 



Description — This genus is represented by an internal cast of a part of a left valve 



Superfamily Pectinacea 



Family Pectinidae 



Genus Aviculopecten M'Coy 



Amculopecten ? umbonata (Hall)? 



Plate 54, figures 7, 8 



Avicula umbonata Hall, 1859, Pal. N. Y., vol. III. p. 284, pi. LI, fig. 6, 1861. 



Description — "Shell rhomboid, nearly once and a half as long as high; anterior 

 margin broadly rounded; posterior side narrower and acutely rounded, very convex 

 in the middle and upon the umbo; anterior wing small, acute, extending a little beyond 

 the anterior margin of the shell, and separated from the body of the shell by a narrow 

 well-marked sinus; posterior wing larger, separated from the body of the shell by a broad 

 defined sinus. Surface marked by concentric lamellose striae, which are closely crowded 

 on the wings. 



" [Remarks] — The specimen has nearly all the shell exfoliated, and the small portion 

 remaining is somewhat imperfect, the cast retaining the impressions of the lamellose 

 striae "—Hall, 1859 



Remarks — Doubtfully included here are two portions — the greater part of one is 

 exfoliated — of left valves, and an internal cast of a part of a left valve 



In addition to the concentric lines of Hall's specimen, they show concentric wrinkles; 

 and the specimen that best exhibits the shell surface shows two faint, radial ribs. 



