134 BRITISH MOLLUSKS. 



generically speaking, is produced with, so little variation in the 

 animal as in the family of Lymnceacea, and the genera are unusu- 

 ally distinct from each other. In Planorhis, the shell is a slowly 

 increasing tube, coiled rotariwise into a flattened disk ; in Physa 

 and iAjmncea it is an inflated spiral, sharply acuminated ; in An- 

 cylus it has only a tendency to coil in an early stage of growth, and 

 enlarges without convolution like a limpet. The animal is cha- 

 racterized throughout by nearly a similar organization of parts. 

 The respiratory sac, as we have already shown, is both pulmonary 

 and branchial, the head is produced into an obtuse muzzle, and the 

 eyes are situated at the inner base of the tentacles. The parts in 

 which there are differences are the tentacles and the mantle. In 

 "Planorhis and Physa, the tentacles are slender and bristle-like ; in 

 Lymncea they are flatly triangular ; in Ancylus they are slender but 

 short, widely separated, and triangular at the base. The mantle, 

 like the shell which it secretes, is most variable. In Planorhis, it is 

 retained within the shell ; in Pliysa it is reflected over the shell on 

 either side in a curiously digitate lobe ; in a section of Lymncea 

 (Amphipeplea) it is reflected without digitation over nearly the 

 whole of the shell ; in Ancylus it lines the shell, which entirely 

 covers the animal. 



The genera of British Lymnceacea are : — 



1. Planorbis. Animal carrying a horizontal rotary coiled tubu- 



lar shell, tentacles slender and bristle-like. Shell horny or 

 glassy, of from three to seven slowly increasing whorls. 



2. Physa. Animal carrying an ovately inflated sinistrally coiled 



shell, over the edge of which the mantle is reflected in digi- 

 tate lobes, tentacles slender. Shell transparent, shining, of 

 five whorls. 



3. Iiymnsea. Animal carrying a variously shaped shell, over 



which the mantle in one instance is amply reflected, tentacles 

 broadly flatly triangular. Shell varying from ventricosely 

 ovate to elongately turreted, of from four to eight whorls. 



4. Ancylus. Animal carrying a small limpet-shaped shell, 



which entirely covers it, tentacles short, slenderly triangular. 

 Shell cap-like, with the vertex turned subspirally sometimes 

 to the left, sometimes to the right. 



