PTSIDIUM. 69 



should have been mistaken for so many species. P. casertanum, 

 its foreign analogue, to which it bears the closest resemblance, 

 and from which it is very difficult to separate it, is likewise widely 

 distributed and subject to much variation. 



P. abditum is our most common species, and occurs generally 

 in great numbers; its epidermis, though usually light yellow, is 

 at times, according to the habitat of the shell, of a much darker 

 color ; the surface is at times also quite rough and the strice coarse. 



Compared with P. nov-eboracense, to which it is nearly allied, 

 it differs in being less heavy, more rounded, less full, the beaks are 

 less large, the hinge-margin is straighter, the anterior extremity, 

 which in the P. nov-eboracense forms a declivity from the beaks, is 

 more regularly rounded, the distance from the beaks to the basal 

 margin is less great, the exterior surface is much smoother, and 

 the epidermis of a lighter color. 



Pig. 73. 



P. abditum. 



8. Pisidiilin simile, PRIME. Shell rounded-oval, very much 

 elongated, very inequilateral, slightly compressed ; anterior 



side rounded, posterior subabrupt ; beaks moderately full, Fig. 74. 

 raised ; surface smooth, epidermis light straw color ; hinge- 

 margin nearly straight. 



Long. 0.14; Lat. 0.13; Diam. 0.07 inches. 

 Hab. At Guadeloupe, in the West Indies. (Cabinets of 

 Smithsonian Institution, Morelet, and Pjime.) 



This species bears the greatest resemblance to P. abditum in 

 outline and size ; it is, however, a little more elongated, and more 

 compressed. 



9. Pisidium cliilense, DESJIAYES. Shell small, delicate, moder 

 ately inflated, elongated, inequilateral ; anterior side a little 



longer, angular at end, posterior subtruncate ; beaks large, Fig. 75. 

 full, somewhat raised ; striae very fine, surface smooth ; epi 

 dermis light yellow ; hinge-margin nearly straight ; cardinal 

 teeth small, out distinct ; lateral teeth a little elongated. 



Hab. South America, at Valparaiso, and at Coquimbo in 

 Chili. (Cabinets of Smithsonian Institution, Cuming, and Prime.) 



