OF CONCHOLOGY. 197 



posterior slope, with a fine raised line in the middle of the slope ; 

 posterior ventral margin nearly rectilinear. 



This shell is covered with a very thin, pale shining epidermis, 

 and varies greatly from the typical species, P. tenuis. It is in 

 perfect preservation, the 4 specimens in the collection having the 

 valves connected as when living, and not the least abrasion 

 visible. 



P. OBLiQUis, Gabb.— PI. 10, fig. 15. 



As Mr. Gabb has fully described this shell, I have only intro- 

 duced it to show the two valves in connection, of the actual size 

 of the largest specimen. 



P. ERECTUiM, Conrad. — PI. 10, fig. 16. 



Triangular ; both valves ventricose, not oblique ; anterior end 

 oblique, truncated ; posterior side produced, cuneiform, flexuous, 

 extremity angular ; ventral margin rounded ; summits very pro- 

 minent ; cardinal tooth comparatively small. 



There is only one specimen of this graceful species, the largest 

 of the genus known. The valves are much less unequal than 

 in the preceding species, and the erect beaks give it a very dif- 

 ferent contour from the other species. The character oblique 

 should be omitted from the generic diagnosis. 



P. CUNEATUS, Conrad. — PI. 10, fig. 12. 



Triangular, oblique, ventricose, solid, subequivalved ; beaks 

 terminal, summit very prominent and oblique ; anterior end ab- 

 rupt ; posterior end subtruncated ; disk somewhat flattened me- 

 dially ; umbonal slope rounded, undefined, nearly marginal ; ven- 

 tral margin nearly straight posteriorly ; cardinal tooth oblique. 



P. OVATUS, Conrad. — PI. 10, fig. 4. 



Ovate, slightly ventricose, disk of right valve regularly curved, 

 of the opposite valve slightly flexuuus towards the posterior end ; 

 beaks situated about one-fourth the shell's length from the end 

 margin ; summits narrow, prominent, oblique, anterior end an- 

 gulated, the margin beneath obliquely truncated ; ventral margin 

 rounded ; cardinal tooth directed slightly posteriorly, very erect. 



This shell is somewhat similar in outline to Cytherea eonvexa, 

 Say. It is white, polished and nearly equivalved, and is very 

 unlike the other species in form. 



P. ALTUS, Conrad. Plate 11, fig. 1. 



Description. Cordate, inflated, nearly equivalve ; summits 

 prominent, angulated on the anterior margin ; anterior sub- 

 margin very oblique, slightly concave in outline, angulated, 



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