112 CYCLADIDiE. 



The foreign analogue, P. conicum, from France, is so closely 

 allied to our species that it is with the greatest care only that they 

 may be separated. 



P. compressum is more trigonal and less inflated than P. vari- 

 abile ; it is more equilateral than either P. virg-inicum, Adamsii, 

 or abditum, and more oblique and less equilateral than P. aqni- 

 laterale. 



The animal is remarkable for its liveliness. It is found sparingly 

 during the spring and not at all in winter. It inhabits both still 

 and running water, and buries itself sometimes in the mud (^Prime'). 



Pisidium sequilaterale.* 



Pisidium cequilaterafe, Prime, Bost. Journ. K"at. Hist. vi. 366, pi. 12, figs. 23-25 (1852) ; 

 Ann. N. Y. Lye. vii. 98. 



Shell small, stout, heavy, somewhat inflated, rhomboidal, sub- 

 equilateral ; posterior margin a little angular where it meets the 

 basal margin ; inferior and anterior margins slightly 

 Fig.^21. rounded ; beaks central, large, prominent, rounded, not 

 approximate ; valves very solid, moderately convex, in- 

 terior light blue ; strise fine, surface glossy, epidermis 

 very variable in color, light yellow, greenish, or brown ; 

 p.fpqniintfraie. lnno;e-marp;in curved, cardinal teeth small, lateral teeth 



Enlarged. c & ' ' 



strong, distinct. Length, fifteen hundredths of an inch ; 

 breadth, fourteen hundredths of an inch ; width, one tenth of an 

 inch. 



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

 York. 



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

 quadrangular form ; the latter gives it somewhat the appearance 

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



Compared witli P. variabile, to which at first sight 

 Fig^422. j|. |)gr^pg rj^ general resemblance from the gloss and 



color of its epidermis, it differs from it very materially 

 in not being at all oblique, and in being equilateral ; it 

 ' p. (Bquiiateraie. is also mucli Icss full. Somcwhat rare. I discovered 

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

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



* Sec note, page 107. 



