LIMNEANA. MOLLUSCA. PLANORBIS. 209 



Shell lenticular, light transparent horn-color ; whorls four, 

 flattened so that the width of each is at least twice its depth, the 

 upper and lower surfaces convex, and brought to a sharp exterior 

 edge ; the last half of the outer whorl deflected, so that the 

 termination of the sharp edge is on a level with the lower surface 

 'of the preceding whorl; inner whorls slightly depressed, and 

 somewhat more rounded ; suture moderately impressed ; striae of 

 growth faint ; beneath abruptly umbilicated, displaying the edges 

 of all the whorls within ; aperture very oblique and angular ; 

 edge very sharp, below running forwards a little along the um- 

 bilical edge of the preceding whorl, then crossing obliquely for- 

 wards and upwards, leaving a callus, it passes off again a little 

 below its carinated edge. Longest diameter J inch, shorter di- 

 ameter f\ inch. 



It is found in most brooks, ditches, and margins of ponds, 

 which are permanent through the summer, adhering to sticks and 

 stones. 



This shell has a striking resemblance to the P. font&na of Europe, 

 (Lightfoot, Phil Trans., Ixxvi. pi. 2, f. 1-4. Montagu, Test. Brit., 

 462, pi. 6, f. 6. PI. nilidus, Mull., Turt., &c.,) except that the aper- 

 ture is entirely below the sharp edge, instead of embracing nearly an 

 equal portion on each side, as in that shell. It is allied to P. deflectus, 

 Say ; but in that the whorls are more numerous, the exterior edge 

 much rounded, the umbilical region broader and more shallow, and the 

 labrum also embraces but half of the lower surface of the preceding 

 whorl. Were it among the land shells it would be a most unequivocal 

 CAHOCOLLA. 



I cannot but think that the name under which this shell appears in 

 the " Journal of the Academy," is not exactly as was intended by the 

 author, as it is neither a Latin word nor a Latin termination. Sup- 

 posing that by a typographical error, an o has taken the place of a t, 

 we have a legitimate term, and one very expressive of the form of 

 the shell. 



PLANORBIS PARVUS. 



Shell very much compressed, almost equally concave on both 

 sides ; whorls four ; surface nearly smooth ; aperture rounded. 



FIGURE 139. 

 27 



