50 PROCEEDINGS OF THE ACADEMY OF 



DESCRIPTIONS AND ILLUSTRATIONS OF GENERA OF SHELLS. 



BY T. A. CONRAD. 



CRASSATELLA, Lam. 

 Subgenus Pachyth^ertjs. 

 C. Pteropsis, Conrad, PI. I., fig. 1. Cretaceous. 

 C. Ripleyana, Conrad, fig. 7. Cretaceous. 

 C. ligeriensis, D'Orb. fig. 10. Cretaceous. 



I do not suppose this group of Cretaceous, Eocene, and Oligocene 

 shells will be recognized as a genus distinct from Crassatella, 

 and therefore I propose it as a subgenus to mark the differences 

 which characterize the species of Cretaceous and older tertiary 

 formations, and distinguish them from Miocene and recent forms. 

 For description of the subgenus, see Anier. Journ. of Conch., vol. 

 v. p. 47. 



Crassatella. 



Miocene and recent. 

 C . undulata, Say, Plate I., fig. 9. 



PLEUROCONCHA. 



Shell radiately ribbed, hinge of right valve with 2 cardinal teeth, 

 posterior one immediately under the apex, triangular, thick, and 

 directed obliquely posteriorly, anterior tooth approximately direct, 

 no cartilage pit as in Crassatella. 

 Crassatella Gallieni, D'Orbigny. 



. I originally proposed this genus under the objectionable name 

 of Radioconcha, and as it has not been accepted I feel at liberty 

 to alter the orthography though not the meaning of the name. 



In an attempt at a natural classification of bivalves it would be 

 wrong to include this shell in the genus Crassatella, as the above 

 diagnosis must render evident to any one who studies the subject. 



In indicating the genus an error occurred in quoting the name 

 of Gue?*angeri, instead of Gallieni as I intended. It seems hardly 

 possible that any one could suppose that I would quote the As- 

 tarte figured on the same plate as the type of the genus. Had 

 I done so it would have been quoted Astarte, not Crassatella. 

 C. Robinaldini was incorrectly referred to this genus. 



Ptychomya, Ag., is described as having 3 diverging cardinal 



[June 25, 



