INVERTEBRATE PALEONTOLOGY. 241 



been sometimes regarded as constituting as many distinct genera ; hut which 

 appear to me not more than separable as subgenera. They may be sepa- 

 rately defined as follows : 



1. corbula, Brug. (typical). 



Shell varying from oval to subtrigonal or subglobose, with but 

 slight obliquity of the beaks or cartilage-process; very rarely with 

 even the slightest traces of a posterior lateral tooth. Type as stated 

 above. — (Strictly marine.) 



2. anisorhynchus, Conrad. 



Shell nearly or quite equivalve, transversely pyrifonn, the poste- 

 rior side being rostrate; beaks nearly equal, and distinctly incurved ; 

 hinge, muscular, and pallial impressions exactly as in Corbula proper, 

 excepting that the cardinal tooth is furrowed. — Corbula pyriformis, 

 Meek. (Associated with fresh and brackish water types.) 



3. pachyodon (z= Pachydon, Gabb, and Anisothyris, Conrad). 



Shell variable in form, generally with obliquely subspiral beaks; 

 cardinal process very oblique ; sometimes with a rudimentary poste- 

 rior lateral tooth. — P. obliqua, Gabb. (Associated with marine and 

 estuary types.) 



In regard to the propriety of including Pachyodon and Anisorhynchus as 

 subgenera under Corbula, there may be different opinions. It is worthy of 

 note, however, in this connection, that the names Pachyodon (or Pachydon ) 

 and Anisothyris were proposed by Mr. Gabb and Mr. Conrad for the first, 

 under a misapprehension in regard to the cartilage of the typical species ; 

 that is, both of these gentlemen supposed that these shells had a small 

 external ligament, and no internal cartilage ; which would, of course, be a 

 good generic distinction. On examining a full series of nearly or quite all of 

 the described species of this group, sent by Professor Orton to the Smith- 

 sonian Institution, I saw that soine of the valves have a little furrow in the 

 cardinal margin near the beaks, that presents very much the appearance of 

 being for the reception of a small external ligament. Other individuals, how- 

 ever, showed nothing of the kind; and on examining specimens with the oppo- 

 site valves united, it became clearly evident that this little furrow, when 

 present, is merely for the reception of the slightly-lapping dorsal edge of the 

 opposite valve; so that there is really not the slightest space for even a very 

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