PLANORBIS. 79 



awaits the return of moist and more congenial weather. 

 All of them frequent stagnant or slowly-running water, 

 and are herbivorous. Their eggs are enclosed in a 

 globular bag, which is fixed to stones and the stalks or 

 leaves of submerged water-plants. Owing to the sluggish 

 habits of most of the species, as well as to the nature 

 of their habitat, the surface of their shells is apt to 

 become encrusted with a mineral or vegetable deposit. 

 The genus seems to have been originally indicated by 

 Petiver in his ' Gazophylacion.'' 



There being several species, it may be more con- 

 venient to divide them, as before, into sections. 



A. Shell glossy ; last whorl very large in proportion to the 

 rest, and partly covering the preceding one. 



1. Planorbis linea'tus*, Walker. 



Helix lineata, Walker, Test. Min. Ear, p. 8, pi. 1 . f. 28. P. laacstris, F. & H. 

 iv. p. 162, pi. cxxviii. f. 1-3. 



JioDY reddish-brown, tinged with violet, dark purple in 

 front with a few black specks : head strongly bilobed : ten- 

 tacles filiform, diverging at their base : eyes small, but distinct, 

 scarcely prominent : foot broad and rounded in front, gradu- 

 ally narrowing and pointed behind. 



Shell quoit-shaped, the upper being rather more convex 

 than the lower side, both sides depressed or almost concave in 

 the middle, rather thin, very glossy, semitransparcnt, reddish 

 or yellowish-horncolour, or grey, closely striate in a trans- 

 verso direction, the striae becoming curved towards the mouth : 

 epidermis very slight : periphery (or circumference of the 

 outer whorl) bluntly keeled: luhorls 4, compressed on all 

 sides ; the last exceeding in size the rest of the shell, and con- 

 cealing nearly two-thirds of the preceding whorl in its clasp : 

 suture distinct, but not deep : mouth obliquely heart-shaped : 

 outer lip thin and flexuous, obtusely angulated above : um- 

 hilicus small and rather deep : inside the last whorl are from 



* Streaked. 



