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 (Id. 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. Dali 
* Cooke, A. H., “Molluscs,” p. 24. 
