214 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



The first of these groups he subdivided into the following smaller sec- 

 tions: (1) the Multicostatce, represented by such forms as P. multicostata 

 and P. semicostata, Ag , and ranging from the Jurassic to the Cretaceous; (2) 

 the Trigonatee, represented by P. arcuata and P. undo, Ag., ranging from 

 the Cretaceous to existing seas ; (3) the Bucardince, represented by P. 

 cincta and P. dccussata, Ag., ranging from the Lias to the Tertiary. 



The second primary division, he subdivided into (1) the Flabellatce, 

 represented by P. pelqgica and P. similis, Ag., and apparently confined to 

 the Jurassic; (2) the Ovalaires, represented by P. tenuicostata and P. 

 pectinata, Ag., found in the Jurassic ; and (3) the Cardissoides, represented 

 by P. cancellata and P. cardissoides, Ag., also found in the Jurassic. 



I have thought, however, that the arrangement might be simplified by 

 disregarding, at least as a primary distinction, the presence or absence of 

 a false cardinal area (in which character there seem to be numerous inter- 

 mediate gradations, from species in which it is wanting, or only very slightly 

 marked between the beaks, to others having it well defined along the whole 

 hinge margin*), and grouping the subordinate sections as follows: 

 1. pholadomya, Sowerby (typical). 



a. (zr Trigonatee (part), Ag.). — Shell generally transversely 

 oval or oblong ; both extremities gaping, or with anterior sometimes 

 closed; beaks depressed or moderately prominent, sometimes the 

 point of one indenting the other ; surface with concentric ridges often 

 extending upon the extremities, and crossed on the flanks by well- 

 defined radiating costae ; false area obscure or wanting. — Type as 

 stated above. (Cretaceous, Tertiary, and recent.) 



b. (zz Multicostata. and Flabellatce, Ag.). — Shell transversely oval, 

 oblong, or more or less elongated ; beaks rather depressed ; radiating 

 costae well marked, scattering and prominent, or smaller and more 

 numerous, generally confined to the flanks ; false are well defined or 

 obsolescent. — P. semicostata and P. pelagica, Ag. (Jurassic and 

 Cretaceous.) 



c. (— Ovalaires, Ag.). Shell transversely oval or oblong, rather 

 compressed or moderately convex; gaping at one end; costai linear, 

 and often eremite; false area well defined. — P. tenuicostata and P. 

 pectinata, Ag. (Jurassic.) 



•This character, if we give it prominence, also separates widely, species that are closely allied iu 

 all other respects. 



