116 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



surface concentrically striated or furrowed; margins closed, crenate or 

 smooth within ; lunule well defined ; hinge strong, provided with two, more 

 or less diverging, generally striated, cardinal teeth in each valve, and an internal 

 cartilage-pit behind them; lateral teeth usually one in each valve, the posterior 

 more or less nearly parallel to the hinge-margin in the right valve, and fitting 

 into a corresponding groove in the other, with a smaller elongated tooth 

 below it; a small anterior lateral sometimes occurs in right valve, fitting 

 into a corresponding pit in the left; muscular impression and simple pallial 

 line strongly defined ; anterior pedal scar distinct from that of the anterior 

 adductor ; no external ligament. 



The older fossil species of this genus present, in some instances, more 

 or less marked differences, from the Miocene and recent, in form as well as 

 in the details of the hinge. On these characters, Mr. Conrad has proposed to 

 divide the genus into two subgenera, nearly as stated below.* 



1. crassatella, Lam. (typical). 



Shell with cartilage-pit behind cardinal teeth comparatively 

 small ; free margins not crenate. — (Type as above. Recent and 

 Miocene.) 



2. PACHYTHiERUS, Colliad. 



Shell with cartilage-pit behind the cardinal teeth generally large, 

 free margins crenate within. — C Vindinnensis, d'Orbigny. (Eocene 

 and Cretaceous.) 



These distinctions, however, scarcely seem to be of sufficient importance 

 to warrant the arrangement of the species under distinct subgeneric names. 



Professor Agassiz separated, under the name Ptychomya, with Crassa- 

 tella Robinaldina, d'Orbigny, as its type, a group thought by some not to be 

 generically distinct from Crassatella. It differs, however, in having three 

 diverging cardinal teeth in each valve, the posterior ones being very long, 

 and double in the right valve, with the cartilage-pit placed in front. It is 



* Mr. Conrad has proposed a genua Badioeoncha (= Pleuroeoncha) in the Am. Jour. Couch., V, 47, 

 and Proceed. Acad. Nat. Sci. Puilad., 1872, 50, with Crassatella Gallieunei, d'Orbigny, as the type, that seems 

 to be closely allied to Crassatella. It is radiately costate, however, and is said to have no internal carti- 

 lage-pit. If the latter character really exists, it must be generically distinct from Crassatella; if not, 

 however, the radiating eosta? would hardly be of more than subgeneric importance. Mr. Conrad says 

 that in lirst proposing this geuus it was by inadvertence that he cited under it Jstarte Guerangeri and 

 Crassatella Robiualtliua, d'Orbigny, instead of C. Galliennei. The first is an Jsfar(e-like form, that he 

 makes the typo of a new geuus, Plionema; while the second is tin' typo of Ptychomya, as stated above. 



