46 MICHIGAN ACADEMY OF SCIENCE. 



bodies of water exist. And, in the absence of these, they seem often to 

 overlap along their borders, owing to the eccentric distribution of many 

 species, resulting from peculiar local conditions of climate, temperature, 

 etc. In the main they are well characterized by the peculiarities of their 

 respective faunas. Thus in regard to the land shells the Northern Kegion 

 is entirely deficient in the larger Helices, which seem unable to with 

 stand the extreme vigor of the climate, and is peopled by a multitude of 

 smaller forms such as the Zonitidw and Pnpidre. whose greater tenacity of 

 life has enabled them to occupy an enormous territory to the exclusion of 

 their larger and more sensitive brethren. Not only have these genera 

 possessed to themselves the entire Boreal Kegion proper, but they have 

 extended south in all directions. Many of them are now cosmopolitan 

 in the broadest sense of the word, having an almost world wide dis- 

 tribution, while others, following the lines of the great mountain chains, 

 have found congenial homes all along the extent of the eastern and 

 western highlands. That the extension of these forms into the southern 

 provinces has been from the north rather than from the east or west 

 is shown by the fact that in neither direction have they been accompanied 

 by the species peculiar to the eastern and western regions, as there is 

 little doubt they would have been had this latter hypothesis been true. 



The Interior Region, which as above limited includes the greater part 

 of the United States, is characterized by a large and abundant fauna, 

 both in species and individuals, which is purely indigenous in its char- 

 acter. With the exception of a few species which have effected a lodg- 

 ment in some of the West Indian islands, or wandered southerly into 

 Mexico, and a stray colony located in the Californian and Central 

 Regions in the northwestern part of the United States, the great genus 

 knowm as "Pohjfjyra" is peculiar to eastern North America. Its species 

 are essentially forest loving and consequently rapidly diminish as the 

 deciduous forests disappear toward the north, and with the exception of 

 a limited number of the more hardy forms it is not represented in the 

 dry arid regions of the western states. 



The Californian Region is the home of a large and beautiful group of 

 species very different in appearance from the somber-colored denizens 

 of our eastern forests, and strikingly similar to the Helices character- 

 istic of the northern parts of Europe and Asia on one hand, and of 

 Central America and Western South America on the other. 



The Central Region, which includes the dry and elevated region lying 

 between the Rocky and Sierra Nevada ranges of mountains is. as might 

 be expected from its physical peculiarities, very destitute in the number 

 of its mollusca. But these, such as they are, barring the small forms, 

 which have crept in from the north and some of like genera, but speci- 

 fically peculiar to the Californian Region, which have been able to 

 surmount the mountain barriers, are generically related to forms pecu- 

 liar to the Interior Region. They are not true Helic'ulcr. however, but 

 belong to a more primitive stock, of wide range through the entire 

 Boreal Region. Neither the peculiar Helices of the Californian Region, 

 nor the authocthonous species so widely distributed through the 

 Interior Region, seem to have been able to surmount the mountain ranges 

 which separate it on either side. 



In regard to the existing fluviatile fauna of North America, while in 

 the main the regional limits above defined hold good, yet in many cases 



