3 88 
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 ( Viviparidce ) 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 
Neothauma, 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 (F contecta ) is here figured. The Valvatidce 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. 
