50 MICHIGAN ACADEMY OF SCIENCE. 



to show the existence of three separate faunas, which though in their 

 present distribution more or less overlap each other, nevertheless are 

 essentially distinct. 



Thus we find the Boreal Region, which is substantially coincident with 

 British North America, while lacking almost entirely the larger and 

 more highly organized Helices, the Viviparidce, the Plmroceridw and 

 the characteristic types of North American Unionidce, is the metrop- 

 olis of the Zcmitidce, Pupidce and Succineidce among the land shells, 

 the Limnceidw and Physidce comprising the fresh water pulmonates 

 the Rissoidce and Valvatidce in the operculates and the Cyrenidce in the 

 bivalves. These families comprise the greater 'majority of the smaller 

 species of mollusca represented in our fauna and, as already stated, 

 from this broadly extended home in the north, aided no doubt by their 

 hardy nature and greater vitality and consequent greater ability to 

 adapt themselves to vicissitudes incident to changes of environment, 

 many of them have succeeded in establishing themselves in nearly every 

 portion of the continent. Associated with these groups are certain of 

 the Unionidce, which ranging nearly across the continent in the north 

 do not occur south of the great lakes west of the Alleghaney mountains, 

 but east of that range occupy the entire Atlantic drainage to the almost 

 entire exclusion of other forms. This apparent extension of the Boreal 

 Eegion along the entire Atlantic coast, may at first sight seem anomalous. 

 But when it is considered that these mountains, while offering no ob- 

 stacle to immigration from the north, have formed an almost insurmount- 

 able barrier to the incursion of the species of the Interior Region from 

 the west and that thus the region has been left open to the exclusive 

 occupation of the northern forms, the explanation is so obvious as to be 

 almost self demonstrative. In the same way we find that west of the 

 Sierra Nevada along the entire Pacific coast, a peculiar fauna, which 

 apparently for the same reason has never extended itself toward the 

 east. Excluding the species which have crept in from the north, the Cali- 

 fornian fauna in many of its features is quite as different from that of 

 the eastern portion of the continent, as that is from the fauna of northern 

 Europe. 



The fauna of the Central Region both land and fluviatile, is too sparse 

 almost to be considered. Its claims to regional distinction are based 

 almost wholly upon its negative rather than any positive characteristics. 

 It is wanting in the distinctive forms, both of the Californian and Interior 

 Regions. Its distinctive land shells belong to a group characteristic of 

 the Boreal Region. A few fresh water species are peculiar and are prob- 

 ably the last existing local remnant of the abundant fauna which existed 

 there in tertiary times. 



The Interior Region, lying between the great eastern and western 

 mountain ranges, is the only one which, in its mollusca, exhibits any 

 of the peculiarities of a great continental fauna. Here are found the 

 exclusively North American genera of Polygyra among the land species 

 and Campeloma, Tulotoma and the several genera of the Pleuroceridce 

 among the fluviatile univalves and the extraordinary development of the 

 Unionidce already mentioned. The greater part of this enormous fauna 

 is found south and east of the Ohio and Mississippi rivers. A few of 

 the hardier species of Polygyra have extended north to the limits of the 

 deciduous forest, while the more favorable conditions of temperature 



