3358 



THE MOLLUSKS OR SHELLFISH 



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 narrow behind. The species Vivipara, as their name implies, are vivip- 

 arous, 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 bottom of rivers 

 or canals, and feed upon decaying vegetable or animal matter. The North-Ameri- 

 can species form the genera Melantho and Tylotoma, and most have more solid 



VIVIPAROUS POND SNAILS, Vivipara; male on left, female on right, young 

 (magnified) in front. 



shells than in Vivipara. One of the finest species occurs in L,ake 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 characteristic of the lingual teeth 

 in this family is shown in the figure on p. 3359 representing a single transverse 

 series. Two species occur in England, one of which ( V. conteda} is here figured. 

 The Valvatidce comprise a few small mollusks 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. pisdnalis occur in places throughout the British Isles. The 



