PISIDIUM. Ill 



Pwdinm cirer, Prime, Ann. N. Y. Lvc. vi. 65 (1853), pi. 1, fig. 1. 

 Pisum rompressum, Desha yes, Biv. Brit. Mus. 282 (1854). 

 Pisiiin altile, Deshayes, loc. sup. cit. 280 (1854). 

 Miiscidium compi-essuin, Adams, lice. Gen. ii. 451 (1858). 

 Miisculium altile, Adams, loc. sup. cit. ii. 451 (1858). 

 Miisculium deer, Adams, loc sup. cit. ii. 451 (1858). 

 Pisum deer, Adams, loc. sup. cit. ii. GGO (1858). 



Shell solid, very oljlique, trigonal, triangular, sub-equilateral, 

 very much 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, regular ; 

 epidermis very variable, yellow, gray or chest- 

 nut color ; valves solid, varying in inflation, in- 

 terior light blue ; hinge thick ; cardinal teeth ^ compressum. 

 small, rolnist, compressed, disposed in the shape 

 of the letter V reversed ; lateral teeth distinct, short, strong, placed 

 at an obtuse angle with the hinge proper. Length, sixteen hun- 

 dredths of an inch; breadth, fourteen hundredths of an inch; width, 

 nine hundredths of an inch. 



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

 Pennsylvania, Ohio, and California, and at Montreal and Quebec in 

 Canada. 



This species, though perfectly distinct and well characterized, is 

 sulTiject to much variation ; its very oblique shape is constant ; in 

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

 markably obese ; the young are generally more elongated and more 

 compressed. 



One of the peculiarities of this species, wdiich, however, is at times 

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

 shape of the apex of the beaks, Avhich assume the appearance of 

 wings placed on the summit of the umbos. 



^ ^ , . Fig. 420. 



P. cicer^ from Greenwich, which I place with this 

 species, differs a little from the type of P. compres- 

 sum, in being larger, more inflated ; the beaks also are 



' '^ '^ ' _ p. compressum. 



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. compressum. 



* See note, page 107. 



