958 FRESH-WATER BIOLOGY 



by effecting a lodgment in the bottom, drag the animal slowly 

 forward. The lamellibranchs breathe by means of two gills sus- 

 pended on each side of the body, which are divided into a series of 

 water tubes by septa or lamellae, through which the water circu- 

 lates by means of cilia. The whole body is enclosed in a soft 

 mantle, which secretes the shell along its outer margins. Poste- 

 riorly the mantle has two openings, through the lower of which 

 the water enters the shell, passes forward, aerating the gills and 

 carrying food to the mouth, and then flows out through the upper 

 opening. 



A more detailed account of the structure of the two classes rep- 

 resented in our fauna will be found under the head of classification. 



THE NORTH AMERICAN FAUNA 



As would naturally be expected from the vast extent of the ter- 

 ritory included in the United States and British America and the 

 great diversity both in the climatic and physical conditions prev- 

 alent in different portions of the continent, the fresh-water fauna 

 of North America is not only one of great abundance, both in 

 species and individuals, but also of great diversity in character; and 

 a very large proportion of the genera represented are peculiar to 

 it. While but one of the eighteen families represented in our fauna 

 is peculiar to the continent, that one, the Pleuroceratidae, is extraor- 

 dinarily developed both in genera and species and, where found, 

 its members usually occur in great abundance. 



On the other hand, of the eighty-six genera recognized at the 

 present time, no less than forty-nine, or five-ninths, are peculiar 

 to North America, while of the many hundred of described species, 

 it is safe to say that more than ninety per cent are not found else- 

 where. Indeed, barring the comparatively small number of cir- 

 cumpolar species in the north, and the somewhat larger represen- 

 tation of tropical or subtropical forms found on our southern borders, 

 practically the whole of our molluscan fauna may be said to be 

 distinctively North American. 



The distribution of the various families, genera, and species 

 represented in our fauna varies greatly in the different sections of 

 the continent. 



