26 PHYSA HETEROSTROPHA. 



a spring connected with the Susquehanna, which 

 river P. gyrina does not inhabit. The posterior 

 extremity of the labrum, is never suddenly in- 

 curved to meet the body whirl, as in P. gyrina. 

 I at one time believed (and described) the 

 variety fontana as distinct, on comparing the 

 soft parts with Tieterostropha proper, but I have 

 since found the two gradually passing into each 

 other. The growth of this variety is no doubt 

 retarded by the coldness of the springs it in- 

 habits; which causes the death, in a few days, 

 of those brought from ponds or streams. I 

 have noticed in a spring upon my premises, 

 that the individuals which live upon the stones 

 of a small ripple, are about twice the size of 

 those upon the muddy bottom of the quiet 

 water, two or three yards off. It must be this 

 variety which is sometimes cited as 'P. fonti- 

 nalis,' for the shells of the two species resemble 

 each other very closely. The mantle, how- 

 ever, is not as large, nor the tentacles as thick 

 as in P. fontinalis. 



EXPLANATION or PLATE 1. 



Figure 1 represents the digitated mantle of the right 

 side; 2, the ova; and 3, a dorsal view of the animal. 



