Fig. 65. 



P. cequilaterale. 



64 AMERICAN CORBICTJLAD^E. 



angular where it meets the basal margin ; inferior and anterior margins 

 slightly rounded ; beaks central, large, prominent, rounded, 

 not approximate ; valves very solid, moderately convex, in 

 terior light blue ; strife fine, surface glossy, epidermis very 

 variable in color, light yellow, greenish or brown ; hinge- 

 margin curved, cardinal teeth small, lateral teeth strongj 

 distinct. 



Long. 0.15; Lat. 0.14; Diam. 0.10 inches. 

 Hab. North America, in the States of Maine, Massachusetts and New 

 York. (Cabinets of the Boston Society, Smithsonian Institution, Lewis, 

 Jay, and Prime.) 



Pisidium xquilaterale, PRIME, Bost. Jour. VI, 1852, 366, pi. xii, f. 23-25. 



This species is remarkable for its solidity and for its short and 

 quadrangular form, the latter gives it somewhat the appearance 

 of a Sphserium ; it is the most equilateral Pisidium I know of. 



Compared with P. variabile, to which at first sight it bears a 

 general resemblance from the gloss and color of its epidermis, it 

 differs very materially in not being at all oblique, and in being 

 equilateral ; it is also much less full. Somewhat rare. I dis 

 covered it in the spring of 1852, in a clay pit in the neighbor 

 hood of Augusta, Maine, in company with P. compressum. 



Fig. 66. 



P. cequilaterale. 



4. Pisidium compressum, PRIME. Shell solid, very oblique, 

 trigonal, triangular, subequilateral, very much 

 Fig. 67. drawn up in the region of the beaks, inflated 



in adult ; anterior side a little longer, narrower, 

 produced at the end, posterior broader, sub- 

 truncate ; beaks placed a little posteriorly, 

 small, raised, with a wing-shaped appendage 

 on the summits, distant ; striae distinct, regu 

 lar ; epidermis very variable, yellow, gray or 

 chestnut color ; valves solid, varying in infla 

 tion, interior light blue ; hinge thick ; cardinal 

 teeth small, robust, compressed, disposed in 

 the shape of the letter V reversed ; lateral 

 teeth distinct, short, strong, placed at an obtuse angle with the hinge proper. 



P. compressum. 



