154 HELTCW^. 



obstacle in the way. It also retracts occasionally, 

 and apparently without any reason, one of its horns 

 and not the other. It does not appear to be ovo- 

 viviparous like the next species (P. jcmbiiicata)!' 



In immature specimens the mouth has, in addition 

 to the principal folds on the base of the penultimate 

 whorl and pillar, transverse folds (similar to the plaits 

 in Planorbis lineatus), which are placed at a short 

 distance from one another, at a right angle to the 

 position occupied by the folds on the outer lip of 

 full-grown specimens, and are distinctly visible outside 

 the shell. 



Var. pallida. — Shell of a lighter colour, sometimes whitish. 



3. P. umbilica'ta,* Draparnaud. Pl. IX. 



Body elongated, strongly truncate in front, greyish-brown 

 above, of a hghter colour underneath, head and neck speckled 

 with black ; mantle roundish, covered with minute black and 

 white specks ; tentacles greyish-brown, finely spotted with 

 black, upper pair rather thick, very close together at their base, 

 bulbs large, lower pair very thick, widely diverging ; foot rather 

 broad, rounded at both extremities. 



Shell subcylindrical, somewhat thin, glossy, semitransparent, 

 brownish or yellowish horn-colour with faint, irregular, close-set, 

 obhque striae in the line of growth ; periphery rounded, occa- 

 sionally with an indistinct blunt keel ; epidermis thin ; whorls 

 6-7, convex ; spire short, apex obtuse ; suture distinct, but 

 rather shallow and oblique ; mouth horseshoe-shaped, with a 

 short, slightly curved denticle on the base of the penultimate 

 whorl close to the upper extremity of the outer lip, the pillar is 

 also thickened by an oblique fold ; outer lip broad, considerably 

 reflected and thickened, white, sometimes tinged with pink or 

 reddish brown ; inner lip thinly spread on the base of the 

 penultimate whorl ; umbilicus small, oblique. 



* Having an umbilicus. 



