56 MICHIGAN ACADEMY OF SCIENCE. 



western hemisphere seems to be justified by the latest and best scientific 

 opinion. 



The belief that this invasion was long antecedent to that which later 

 gave rise to the present fauna of the Pacific slope, is based upon the fact 

 that the structural peculiarities of the group are of a more primitive 

 type than belongs to the later invaders and that its present universal 

 range through the Carribean region indicates that it was "an older 

 faunal element" and was in position to take advantage of certain earlier 

 continental extensions, which ceased to exist before the period of the 

 later immigration. 



The second Asiatic invasion is supposed to have occurred in the early 

 part of the Eocene period. Its members belong to a more specialized 

 type of molluscan development, and hence presumably of later origin. 

 Passing over the Behring bridge, it traveled south, leaving along its track 

 the ancestors of the present west coast fauna. The presence of the 

 Mesozoic sea at that time and later the mountains and arid regions 

 of the central province, have hitherto effectually prevented any advance 

 toward the east. The southern extension of this tribe has been essen- 

 tially the same as its predecessor. From it has decended a very large 

 part of the existing fauna of central and southern America and the West 

 Indian Islands. 



As has already been intimated, the continental elevations which united 

 the Greater Antilles with Central America undoubtedly afforded a land 

 bridge between Cuba and the then islands of Florida, which gave the 

 handful of the tropical species now found there an opportunity to pass 

 into that region and spread as far north as climatic conditions would 

 allow. That this invasion was comparatively recent, is shown by the 

 small amount of differentiation which has taken place between the 

 Floridan and Cuban forms. 



The helicoid fauna of the eastern part of the continent is composed of 

 two elements, both comparatively simple in organization and undoubtedly 

 of great antiquity. 



The one, comprising the patuloid snails, is probably nearest to the 

 primitive type of all existing forms. It is, as might be expected, of almost 

 universal distribution in all parts of the world, and on this continent, 

 while its representatives are found in all the provinces, occupies the cen- 

 tral region to the exclusion of the forms so abundantly developed both in 

 the east and along the Pacific. 



The other, the Polyqyrw, is wholly confined to America and is be- 

 lieved to be one of the few remaining races of the earlist forms of 

 helicoid life. Although the paltBontological history of the group is very 

 scant, there cannot be much doubt that its ancestors have occupied 

 eastern American soil ever since it had a fauna of Helicidw. The same 

 barriers which operated to prevent the eastern extension of the successive 

 invasions of the Pacific coast from the old world, in all probability have 

 been the means of preserving these native races from what might have 

 proved a fatal competition with the more highly organized invaders 

 from the west. 



Of the source and method of distribution of the pulmonate fresh water 

 mollusca little can be said. There has not been so much splitting up into 

 families and genera as has occurred on the terrestrial forms. The 



