FISHES, MOLLUSCA, AND OTHER OBJECTS. 38 



are to be found in similar situations. The coiled 

 Hhells (Paludina) cannot be mistaken, especially the 

 biggest, P. vivipara, which flourishes most in large, 

 slow-moving rivers. We have known this species 

 to be collected in the dykes of the Norfolk rivers, 

 and sold and eaten as marine periwinkles ! The 

 teeth of all the fresh-water mollusca are charming 

 objects when prepared for the microscope. Perhaps 

 the most complex dental structure of this kind is 

 that possessed by the Neritina fluviatilis- -a beau- 

 tiful little zebra-marked shell, with mouth half- 



Teeth of Neritina jiumatilis. 



closed, to be found adhering to the piles, under 

 water, in many of our larger rivers. In the lingual 

 teeth of this species, the central tooth a is minute ; 

 the teeth of the lateral areas (pleurai), termed 

 uncini, are about sixty in number. The first is 

 very large, and of a remarkable shape; the rest 

 being slender, hooked, and toothed. As a rule, the 

 teeth of the fresh-water snails are remarkable for their 

 fewness, those of land snails being, on the contrary, 

 marked for their extraordinary number. In marshy 

 spots, such as those bordering a natural pond, may 

 be found the peculiar mollusc Testacella, which in 



o 



