LIMNEANA. MOLLUSCA. PLANORBIS. 207 



more rapidly ; and it maintains its yellowish horn-color, whereas 

 P. albus assumes a spermaceti or still whiter appearance. The lines, 

 too, disappear more entirely when the epidermis is gone. 



Planorbis elevatus. 



8hell small, whorls three or four, swelling above, with the apex 

 sunken, deeply concave beneath ; aperture slightly oblique. 



State Coll., No. 291. Soc. Cab., No. 2417. 



Planorbis elevatus, Adams ; Bost. Journ. Kat. Hist., iii. pi. 3, f. 15. 



Shell small, light grass-green, translucent, faintly marked with 

 lines of growth ; whorls three and a half or four, the tube not 

 rapidly enlarging, and considerably flattened ; whole shell flat, or 

 shghtly elevated above, the tip depressed so as to form a small pit ; 

 below forming a deep, tunnel-shaped cavity, the whorls presenting 

 an obscure angle as they revolve around it ; suture deeply impress- 

 ed ; aperture slightly oblique, its upper edge on a level with the 

 spire, or very slightly declining ; lower edge descending considera- 

 bly below the level of the under surface ; portion of the preceding 

 whorl embraced by the aperture constituting about one fifth of its 

 circuit. Breadth ^ inch, height -j^ inch. 



Inhabits rivulets and pools in Norfolk and Plymouth Counties. 



This shell is closely allied to P. parvus and P. liirsutus. But the 

 first is a more depressed, discoidal shell, its upper surface more broad- 

 ly and deeply concave, the lower surface very little more concave 

 than the upper, and the aperture much more oblique. P. hirsiitus 

 has a still greater altitude, a very rapidly increasing tube, is deeply 

 concave above and below, its color is lighter, and its lines of hairs, 

 when present, afford a very marked distinction. It may possibly 

 prove to be the immature shell of some other species. 



Planorbis deflectus. 



Shell concave on the left side ; ivhitish horn-color ; whorls four, 

 compressed, sub-carinated, aperture greatly declining. 



Figure 136. 



State Coll., No. 168. Soc. Cab., No. 1266. 



Planorbis deflectus, Say ; Long's 2d Exped., (.Appendix), 2C1, pi. 15, f. 8. 



