PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 611 



II. 



NOTES UPON THE CHARACTERISTICS AND ORIGIN OF 



THE NON-MARINE MOLLUSCAN FAUNA OF 



SOUTH AMERICA. 1 



In the following synopsis I have limited myself to a brief consideration 

 of data derived solely from mollusks. This course is not due to any 

 underestimation of the value of other groups in biogeographic work ; but 

 rather because conclusions drawn from a group known to me at first 

 hand have a certain value which would not attach to borrowed data. The 

 classification used herein is that of the Manual of Conchology so far as 

 the groups have been considered in that work. 2 



It must constantly be borne in mind by those comparing the distribu- 

 tion of non-marine mollusks with that of vertebrate groups, that not 

 only has evolution proceeded more slowly in the former, but migrations 

 have been slower. Thus, when a Pliocene communication was established 

 between North and South America, there was a rapid and extensive 

 invasion of both areas by mammals ; but the molluscan invasion was very 

 much slower and never extended nearly so far. Land and fresh-water 

 mollusks are restrained by conditions which affect mammals and birds far 

 less, such as areas with little forest, unsuitable or very scanty rock on the 

 surface, or short river systems, not well connected. 



Any inquiry into the antecedents of a fauna leads to the question of 

 where its component groups had their rise. The rarity of land and fresh- 

 water shells as fossils, and the great antiquity of the family groups, renders 

 this question very intricate. The origin of many groups is still quite un- 

 known ; yet most of the larger families of land-snails, and a few of the 

 fresh-water groups may be traced back with considerable certainty, if not 

 to definite centers, at least to extensive areas of evolution. 



The several origins of air-breathing gastropods from marine groups 

 from the Opisthobranch stock, the Rhipidoglossa and the Taenioglossa 



1 For bibliography see H. von Ihering, Archhelenis und Archinotis, Leipzig, 1907. A. Ort- 

 mann, Princeton University Expeditions to Patagonia, IV. T. Arldt, Die Entwicklung der Kon- 

 tinente, Leipzig, 1907 ; bibliography on pp. 622-631. 



2 The family groups of land snails almost all differ widely in contents and limits from those 

 of Fischer's Manuel and other systematic works ; a fact of first importance in dealing with the 

 distribution of the groups, and the relationships of faunas. 



