HELICINA IN NORTH AMERICA 157 



Towards the latter part of the Glacial Epoch, when the 

 existing geographical conditions of the northern lands were 

 gradually brought about, the temperature of the Atlantic and 

 Pacific Oceans decreased, causing a diminution of precipita- 

 tion on the continents. With slight climatic oscillations the 

 conditions almost all over the northern hemisphere gradu- 

 ally seem to have grown less favourable for the survival of 

 Tertiary animals and plants than they were during the Ice 

 Age. Warmth and moisture-loving species are almost every- 

 where being replaced by others that can support greater 

 extremes of temperature, and the former only exist here and 

 there in diminishing colonies as relicts of the past. 



In the United States we have evidence of such a course of 

 events, not only among the higher groups such as the 

 mammals; some of the more slowly-moving invertebrates 

 are even more trusty indicators of the past geological history 

 of the country. 



Three species of an operculate snail belonging to the family 

 Helicinidae inhabit the United States. One of them(Helicina 

 chrysocheila) occurs in Texas near the mouth of the Rio 

 Grande. Another (H. orbiculata) has a wide range from' 

 Florida and Texas as far north as Arkansas and Tennessee. 

 A third (H. occulta) lives in isolated colonies among loose 

 leaf-mould in well-wooded districts from Carolina to Wis- 

 consin and Minnesota. Though inhabiting States where 

 severe winter frosts are common, it is amply protected against 

 them by its mode of life. Mr. Cooke* maintains that all 

 operculate land mollusks are exceedingly sensitive to cold, 

 and that the whole group is undoubtedly a product of tropical 

 or semi-tropical regions. This view is borne out by the range 

 of Helicina. Far to the west of North America, beyond the 

 Pacific Ocean, a few stragglers occur in Burma and on the 

 Nicobar islands. As we advance eastward they increase in 

 number in certain parts of southern Asia. Almost throughout 

 Polynesia we meet with some species, and also on the West 

 Indian islands (Fig. 11). That the genus is a very ancient 

 one must be evident from its geographical distribution. It had 

 already reached America in early Tertiary times, for Dr. Dall 



* Cooke, A. H., " Molluscs," p. 24. 



