202 Fresh-Water Shells. 



Its great solidity, and its remarkably broad, corpulent aspect, 

 approximating in character to Paludi'na, cause it to be easily 

 recognised. Its analogue on the European continent is L. i^ereger, 

 which, however, differs from this in being a less solid shell, in 

 having the aperture somewhat expanded, its anterior curve broader, 

 and the fold of the pillar less deep. It comes nearer to L. emar- 

 ginata than to any other American species. 



Genus PLANORBIS, Lam. 



Shell discoidal, whorls apparent above and belotv, ap)erture 

 crescent-shaped, remote fvom the axis of the shell ; operculum 

 wanting ; animal with thread-like tentacula. 



Planorbis Trivolvis. — (Say.) 



^""^i Shell orbicular, yelloAvish-white, 



brownish, or chestnut colour ; umbi- 

 \ licated on tlie right side, cup-shaped 

 ■4 on the left ; on the right side scarcely 

 three volutions, separated by a profound 

 suture, are visible, as they disappear 

 in the umbilicus, their faces, especially those of their interior whorls, 

 being slightly carinated ; on the left side at least four whorls are 

 seen, Avhich, by their faces, form a cup-shaped depression, scarcely 

 distinguished by the suture, except the last half of the outer 

 whorl, on the whole of which a well-marked carina revolves, 

 forming a margin to the cup ; the carina gives the whorl a flat- 

 tened appearance on this side ; surface covered with fine, regular, 

 raised, transverse lines, somewhat grooved between them ; aper- 

 ture sub-ovate, inclining to the right, its right margin more ad- 

 vanced than the left, broadly and regularly rounded ; left lip ab- 

 ruptly angulated where the carina terminates ; lip usually thicken- 

 ed within, and of a reddish brown- colour. Large diameter y'^^j- 

 inch, small diameter, -^^ inch. 



Animal dark-russet or dusky, covered with pale-yellowish dots., 

 Planorhis corpiuUntus of Say seems little else than an exuber- 

 ant growth of this shell. The following differences may be noted. 

 It is at least double, often three times, the size. It is a thinner 

 shell. On the right side the volutions are less compact, and ex- 

 hibit a larger portion of each whorl ; on the left side the suture 

 is more and the carina less distinct ; the aperture is much more 

 expanded, and projects far to each side of the preceding whorh 



