388 



MOLLUSCS. 



furnished with gills, they are more or less amphibious, passing part of their time 

 out of the water. Four are British. 



The viviparous pond-snails ( Viviparidai) in general appearance are not unlike 

 periwinkles, having a longish contractile rostrum and elongate pointed tentacles 

 with the eyes situated upon short projections at the base. There are two neck- 

 lappets, the right forming a rudimentary respiratory siphon. The foot is broad 

 in front and narrowed behind. The species of Vivipara, as their name implies, 

 are viviparous, and the young are brought forth provided with a shell having 

 three rows of bristles, and at once start upon an independent career, like the 

 adult. They are sluggish creatures, and generally live more or less in mud at the 



viviparous pond-snails, Vivipara ; male on left, female on right, young (magnified) in front. 



bottom of rivers or canals, and feed upon decaying vegetable or animal matter. 

 The North American species form the genera Melantho and Tylotoma, 

 and most have more solid shells than in Vivipara. One of the finest species 

 occurs in Lake Tanganyika, and is remarkable on account of the aperture 

 being somewhat effuse at the base, and in having the outer lip slightly sinuated 

 in the middle. On this account it was at one time placed in the separate genus 

 Neoihauma, but examination has shown that it does not essentially differ from 

 Vivipara. The character of the lingual teeth in this family is shown in the 

 accompanying figure representing a single transverse series. Two species occur 

 in England, one of which ( V. contecta) is here figured. The Valvatidaz comprise 

 a few small molluscs closely allied to the last. They occur in Europe and North 

 America, and are remarkable for protruding a plume-like gill beyond the mouth of 

 the shell, which is somewhat discoid or turbinate in form, Valvata cristata and V. 



