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OF THE UNITED STATES. 321 



Cabinet of the Academy and Philadelphia Mu- 

 seum. 



Very common on the southern coast, penetrating 

 compact mud or i lay. Small clods of th:s clay are 

 often rolled ashore by the waves, either containing 

 this species, or exhibiting proofs of having be^n its 

 habitation, by the numerous perforations with which 

 they are distinguished. In many places, where a bed 

 of this mud is bared by the refluent tide, these shells 

 may be seen in considerable numbers, with a por- 

 tion of the smaller side appearing above the surface. 

 It is proportionally broader than the shell figured 

 by Lister, plate 4£3, and it seems to be allied to P. 

 campechensis. 



2. P. truncata. — Shell white, transversely ob- 

 long, sub-pentangular ; anterior margin rostrated, 

 obtusely cuneiform in the middle; posterior margin 

 broadly truncated at tip} valves transversely wrinkled 

 and longitudinally striated, muricated, particularly 

 on the anterior side, with small erect scales, which 

 are not arched beneath ; posterior margin, from a 

 line extending from the beak to the inferior angle of 

 the truncature, destitute of the striae and mutic; 

 hinge callous, foimed of the duplicature of the hinge 

 margin, and destitute of cells, a small tooth upon the 

 inner margin, projecting backward ; dentiform pro- 

 cess curved, prominent, slender, flat. 



Length, three-fourths of an inch. 



Breadth, one inch and seven-tenths. 



Inhabits the southern coast. 

 40 



