42 



7. P. VOR'TEX,* LINN& PL. IV. 



Body rather slender, reddish-brown tinged with violet above, 

 of a lighter shade beneath, indistinctly speckled with black; 

 head very large, much rounded in front; tentacles slender, 

 pointed at the tips, somewhat thicker, and widely diverging at 

 their base ; eyes black ; foot reddish-brown with a yellowish 

 tinge, margins paler, rounded in front, and ending in a tapering 

 and keeled tail. 



Shell greatly compressed, somewhat concave above, flat be- 

 neath, thin, glossy, greyish or brownish horn-colour, regularly 

 and closely marked with fine striae in the line of growth, some- 

 times very faintly striate also spirally; epidermis thin ; periphery 

 distinctly keeled below ; whorls 6-8, somewhat angular, gradually 

 increasing in size ; suture distinct but rather shallow ; mouth 

 nearly oval, slightly compressed, acutely angulated above, in- 

 ternal margin often strengthened by a slender rib ; outer lip not 

 reflected ; inner lip continuous with the other, widely spread 

 over the base of the penultimate whorl ; umbilicus broad, not 

 deep. 



Inhabits the same situations as P. spirorbis, from 

 which it may be distinguished by its thinner and 

 flatter shell, by its distinct and sharper keel, and by 

 the shape of the mouth, which is oval and angulated, 

 instead of nearly circular. 



It is an inactive and irritable species, and delights 

 in floating lazily on the under surface of the water. 

 When, during hot weather, the places of its abode 

 are dried up, it buries itself under the mud and weeds, 

 and after closing the aperture of its shell with a thin 

 white epiphragm, it awaits in a torpid state the return 

 of rain. 



Var. compressa. Shell thinner and flatter, keel more prominent 

 and placed near the centre of the periphery. It is often found 

 with the typical form. 



* A whirlpool. 



