114 



CYCLADID^, 



Musrulium ohscurum, Adams, loc. sup. cit. ii. 452 (1858). 

 Muscidium Kurtzi, Adams, loc. sup. cit. ii. 4.51 (1858). 

 Musculiuin zonalum, Adams, loc. sup. cit. ii. 452 (1858). 

 Pisum ohscurum, Adams, loc. sup. cit. ii. 660 (1858). 

 Pimm Kurtzi, Adams, loc. sup. cit. ii. 660 (1858). 

 Pisum ruhellum, Adams, loc. sup. cit. ii. 660 (1858). 

 Pisum zonalum, Adams, loc. sup. cit. ii. 660 (1858). 

 Pisidium rctusum, Prime, Proc. Zool. Soc. xxviii. 322 (1860). 



Fig. 425. 



P. abditum. Enlarged. 



Shell rouiidcd-oval, elongated, very iueqiiilatcral, moderately con- 

 vex, margins well rounded, beaks placed nearer the posterior side, 



small, slightly raised ; surface smooth, striae 

 not distinct, epidermis variable, generally light 

 straw-color ; hinge-margin very nearly straight ; 

 cardinal teeth small, separa<e, the anterior 

 tooth larger and more prominent ; lateral teeth 

 small, not much elongated. Length, fifteen 

 hundredths of an inch ; breadth, fourteen hun- 

 dredths of an inch ; width, nine hundredths 

 of an inch. 



North America, in New England, in the States of New York, New 

 Jersey, Pennsylvania, Ohio, Michigan, South Carolina, and Califor- 

 nia, in the Lake Superior region, at Montreal in Canada, and in 

 Honduras. 



This species is distributed over such a vast area of country, and 

 varies so much according to the different localities in which it is 

 found, that it is hardly surprising that its numerous varieties 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 striae are 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 (^Prime'). 



Fig. 426. 



P. abditum. 



