LAND REGIONS. 205 



European aiialogne, P. vivipara, but which difters in possessing- 

 four colored bands, whilst the latter has but three. Several 

 species of Limnjiea and Planorbis are identical with European 

 and Siberian species, so that these species have a wide distribu- 

 tion. One of the UnionidEe also, the Margaritana margaritifer-a 

 occurs in all the colder waters of N. America, as well as in 

 Europe and Siberia. 



There are, in the United States, 2 species of Cyrena, numerous 

 Cyclas and Pisidium, and the peculiar genus Rangia or 

 Gnathodon, which inhabits the lagoons of the Gulf States. 



The prodigious multiplicity of individuals of the fresh-water 

 species is a fact as remarkable as the quantity of specific forms. 

 The bottoms of some lakes are formed of such thick deposits of 

 shells that they are gathered for agricultural top-dressing. At 

 Muscle Shoals on the Tennessee River, navigation is actually 

 impeded by the accumulation of Unionidse. The city of Mobile, 

 Alabama, is built upon a bank of Rangia ; the road from New 

 Orleans to Lake Pontchartrain, 6 miles long, is constructed of 

 Rangia shells found at the eastern extremity of the lake, where 

 the bed of shells measures a mile in length, by 200 feet wide, 

 with a depth or thickness of 15 feet. 



Fischer includes the Bermudas in the American rather than in 

 the West Indian region, distinguishing it from the latter by the 

 absence of Cyclostomidse or operculated land shells. Only 19 

 species are known, 9 of which are West Indian and 2 European, 

 the rest peculiar. 6 of these species occur in Texas and Florida 

 but may be considered as acclimatized rather than native forms. 



29. Californian Region. 



The Pacific slope from Alaska to San Diego is inhabited by a 

 fauna ver^^ distinct from that of the American Region. The 

 Helices are highly colored, and more like the European groups 

 and are numerous — principally Arionta, Aglaja, Selenites. The 

 Unionidse and Melanians are very rare, but fresh-water pulmonates 

 abound, and some of them are of Eastern American species. 

 There are no operculated land shells. The principal genera, 

 besides those already enumerated, are Prophysaon, Ariolimax, 

 Binneya, Hemphillia, Pompholyx, Carinfex, Tryonia, Cochliopa, 

 Fluminicola. The Tryonia are all dead shells, occurring numer- 

 ously upon the surface of the ground in the Colorado Desert. 

 Similar shells are found living in Lake Baikal, Siberia. 



30. Canadian Region. 



Fischer has constituted this region for British America, 

 Alaska, Greenland and a part of New England, and states that 

 it is characterized by a mixture of American and European 

 forms. He has divided it into 5 subregions. 



