WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 55 



with the evidence given by their geographic distribution and these are 

 also consistent with the accepted doctrines of geology as to the changes 

 in the earth's surface, which would afford a possibility and opportunity 

 for such distribution, the consensus of all these elements amounts to 

 practically positive proof. With these considerations in mind let us now 

 return to our present fauna and see how far the existing distribution of 

 our mollusca can be explained from the paleontological and geological 

 evidences at our command. 



As already stated the earliest forms of terrestrial Mollusks now known 

 are from the carboniferous deposits of our northern United States and 

 Canada. The genera there represented Pupa and Zonites are indistin- 

 guishable from these genera as they exist today. And it is a notable 

 fact that these genera are now not only universally distributed over this 

 continent, but that they have an almost worldwide range over the globe. 

 The vast antiquity of these forms and their present almost universal 

 distribution must be recognized as correlative facts, the significance of 

 which is obvious. 



The remarkable and peculiar helicoid fauna of the Pacific coast has 

 also been mentioned. These snails are not entirely different from the 

 eastern American fauna in their conchological characters, but in anatomi- 

 cal features as well, and belong to an entirely different sub-family, which, 

 from its more specialized character, is believed to be of much later 

 origin in time, and whose affinities are wholly with the present fauna 

 of eastern Asia. Without going into the evidence upon which the 

 theory is based, it may be stated there is reason to believe during the 

 time when the great Mesozoic sea divided the eastern Archean conti- 

 nent from the western, there were two successive immigrations of 

 helicoid life from Asia over the inter-continental bridge, which then 

 existed across Behring Straits. The first of these occurred at a very 

 early period, probably in Secondary times. Prevented from spreading 

 to the east by the Mesozoic sea, the invading mollusks spread southward 

 along the Pacific into Mexico and Central America. From thence, one 

 division continued south into South America where today it constitutes 

 a large part of the helicoid fauna. About the same time, or as soon 

 as opportunity was ofiforded by the elevation above the sea of the land 

 bridge between Central America and the West Indian Islands (which 

 has undoubtedly existed), another division spread eastward and peopled 

 what are now known as the Greater and Lesser Antilles. In this in- 

 vasion the helices were in all probability accompanied by the ancestral 

 forms of the opercnlated mollusca now so abundant in that region and 

 by a detached colony of the Clausilias peculiar to northeastern Asia, 

 which found a permanent home in the mountains of Ecjuador and Peru. 

 No remnants of this invasion now exist along its line of travel down the 

 Californian coast and with the exception of a few forms, which later 

 passed from the West Indies into southern Florida, none are found in 

 North America north of Mexico. Whether the failure of these mollusks 

 to effect a permanent footing along the Californian coast, was owing to 

 the fact they were exterminaied by some subsequent submergence of that 

 region, or, as suggested by Huxley, that they passed to the south along 

 some continental extension to the west, which is now covered by the 

 Pacific Ocean, cannot now be told. But that was the manner in which 

 one great tribe of mollusks attained in its present distribution in the 



