128 INTRODUCTION. 



diate territory, the most peculiar species of the two are 

 the same. This is an instance of the tendency of similar 

 causes to reproduce the same forms in localities widely 

 separated, though upon the same continent. In it the 

 influences are unfavorable, and the species gradually run 

 out towards the north-east, until, in Nova Scotia, the 

 only one of our larger species common to the more south- 

 ern sections, known to prevail, is Helix alternata, pro- 

 bably the most universally diffused species of our whole 

 catalogue. The number of individuals is also greatly 

 diminished, and they never attain the same size and 

 beauty which they reach in more favorable situations. 

 In many parts of this section, land shells are never seen, 

 and throughout the whole of it they are more or less rare. 

 But, while such is the operation of climate and soil on 

 our native species, Helix hortensis, a species undoubtedly 

 introduced from Europe, has made a successful lodgment, 

 and is the most abundant species throughout the greater 

 part of Nova Scotia, the islands north and east of it, 

 parts of Lower Canada, New Brmiswick, and on several 

 small islands on the coast of Maine and Massachusetts, 

 but does not extend to more southern and temperate 

 parts. The genus Vitrina also belongs to tliis section, 

 as does also Bulimus harpa. 



The distribution of the genera and species in each of 

 the preceding sections is exhibited in the synopsis on 

 pages 128, 129 ; the species supposed to be introduced 

 from foreign countries being printed in italics. The 

 catalogues are, of course, imperfect, and additions and 

 alterations will be made from time to time. 



