PISIDIUM. 65 



Long. O.IG; Lat. 0.14; Diam. 0.09 inclies. 



Hah. North America, in New England, in the States of New York, Penn- 

 sylvania, Ohio, and California, and at Montreal and Quebec in Canada. 

 (Cabinets of the Boston Society, Smithsonian Institution, Jay, Prime, and 

 others.) 



Cyclas ahilis, Anthony, in litt., 1847. 



Pisidium compressum, Prime, Bost. Proc. IV, 1851, 1G4. 



Pisidium altile, Anth. Prime, Bost. Jour. VI, 1852, .353, pi. xi, f. 10-12. 



Pisidium cicei; Prime, Ann. N. Y. Lye. VI, 1853, 65, pi. i, f. 1. 



Pisum compressuvi, Deshayes, Biv. Brit. Mus. 1854, 282. 



Pisum altile, Deshayes, loc. sub. cit. 1854, 280. 



Musculium compressum, Adams, Rec. Gen, II, 1858, 451. 



Musculium altile, Adams, loc. sub. cit. II, 1858, 451. 



Musculium cicer, Adams, loc. sub. cit. II, 1858, 451. 



Pisum cicer, Adams, loc. sub. cit. II, 1858, 600. 



This species, though perfectly distinct and well characterized, 

 is subject to much variation ; its very oblique shape is constant ; 

 in fulness it is exposed to much change, some old specimens are 

 remarkably obese ; the young are generally more elongated and 

 more compressed. 



One of the peculiarities of this species, which, however, is at 

 times wanting, from abrasion or from other causes, is the very 

 singular shape of the apex of the beaks, which assume the appear- 

 ance of wings placed on the summit of the umbos. 



P. cicer, from Greenwich, which I place w^ith this species, 

 differs a little from the type of P. compressum, in being larger, 

 more inflated ; the beaks also are larger, and do not exhibit the 

 winged appendage. P. altile, a mere variety, is more oblique 

 than the type, and does not possess the appendage on the beaks. 

 Both these varieties are darker in color than the true P. com- 

 pressum. 



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

 allied to our species that it is w^ith the greatest care only that 

 they may be separated. 



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

 variabile ; it is more equilateral than either P. virginicum, 

 adamsi, or abditum, and more oblique and less equilateral than 

 P. sequilaterale. 



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



