PISIDIUM. 



115 



Fig. 427. 



P. variabile. Enlarged. 



Pisidium variabile.* 



Ci/clas nitida, Mighels, Lixsley, Amer. Jour, xlviii. 276 (1845). 

 Pisidium variabile. Prime, Bost. Proc. iv. 163 (1851). 

 Pisidium (irande, Wiiittemoke, in litt. 1855. 

 Muscidium variahile, Adams, Rec. Gen. ii. 452 (1858). 

 Pisnm variabile, Ad.vms, loc. .sup. cit ii. 660 (1858). 



Shell heavy, oblique, inequilateral, inflated ; anterior side longer, 

 narrower, somewhat angular at end ; ])Oste- 

 rior subtruncate ; Ijeaks situated posteriorly, 

 full, prominent, not approximate at apex ; 

 valves solid, interior light blue ; stride regu- 

 lar, but very distinct; epidermis glossy, very 

 variable, straw-color or greenish-brown with 

 a vellow zone on the basal margin ; hinge- 

 margin curved ; hinge rather slight ; cardi- 

 nal teeth united, small ; lateral teeth dis- 

 tinct, strong, short. Length, twenty-one 

 hundredths of an inch ; breadth, eighteen hundredths of an inch ; 

 width, seventeen hundredths of an inch. 



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

 Pennsylvania, and Yirginia. 



This species has hitherto always been looked upon l\y collectors 

 as the P. rirg-inicKm ; but having compared it with the original 

 shells, described as Cijclas cliibia, Say, l.)y Dr. Gould, in his Report, 

 and with some specimens of P. virginicum from Westfield, Mas- 

 sachusetts, sent to me Ijy Professor C. B. Adams, as well as with 

 some others sent to me from Philadelphia, by Professor S. S. Hal- 

 deman, 1 have become convinced that it is different from Say's 

 shell. Compared with the young of P. virginicum, it is more ob- 

 lique, less elongated, more inflated, and of a different color. This 

 species is not so elongated as the P. inrginicum ; it 

 is more inflated, the Ijeaks are larger and more tu- 

 mid ; it is also a much smaller shell. Say descriljes 

 Ci/clas dubia as l)eing six twentieths of an inch in 

 length ; P. variabile is only four twentieths of an 

 inch in length, and that it is a full-grown shell 1 

 am led to believe, not only from its heavy striations and mature ap- 

 pearance in general, but also from having found young in the shell. 

 The young is not so oblique as the adult ; it is more elongated, less 



* See note, page 107. 



Fig. 428. 



P. variabile. 



