452 NON-MARINE FOSSIL MOLLUSCA. 



Length, 16 millifneters; breadth, 10 millimeters 5 height, 7 millimeters. 

 The specific name is given in honor of Mr. W. H. Dall, whose impor- 

 tant works upon the mollusca are well known. 



VITRINIDJE. 



It is a suggestive fact that so large a proportion of the fossil non- 

 marine mollusca, which have been found in North American strata, are 

 air-breathing land snails. This result of field-investigation is unex- 

 pected, both because individuals of none of the living land-snails are. 

 found in as great numbers as many of the gill-bearing water snails are, 

 and because all the specimens that have been preserved in the fossil 

 state must have been transported from the land into the waters in the 

 sediments of which they were preserved after the death of the mollusks 

 which formed them. We must therefore conclude that in former geo- 

 logical epochs the land-snails were proportionally quite as abundant as 

 they are now, and it seems probable, also, that the shores of, and the 

 region round about, the Laraime sea, and those of the great fresh-water 

 lakes which succeeded that sea, formed a peculiarly favorable habitat 

 for land mollusca. 



Judging from the character of a portion of the fossil shells that have 

 been discovered, and from the wide diversity in the families of land mol- 

 lusca, which is indicated by the collections that have been made from 

 various formations, we cannot doubt that the family Vitrinida3 was 

 represented, at least during the Laramie and Tertiary periods. It is 

 not to be denied, however, that with a knowledge of the shells only, and 

 they usually more or less imperfect, it is not always possible to say with 

 certainty that those which we refer to that family really belong to it ; 

 or that they may not belong to the Helicidse. 



The three following-named species were described by Meek & Hay- 

 den, and referred by them, not without some doubt, to the Vitrinidae. 

 They obtained them from the Judith Eiver Laramie beds of the Upper 

 Missouri River region, and named them respectively, Vitrinaf obliqua, 

 Hyalina? occidentalis, and H.f evansi.* All three of these forms are 

 represented by figures on Plate 27. 



From the Wind Eiver Group of Eocene strata, in Wyoming, Meek 

 & Hay den also obtained a fine large species, which they described 

 under the name of Macrocydlis spatiosaj which is represented on Plate 30. 



ARIONID2E. 



Among the most interesting remains of land mollusca that have been 

 discovered are those which Dr. J. W. Dawson has published at different 

 times from the Coal-Measures of Nova Scotia. He was the first to demou- 



* U. S. Geol. Sur. Terr. vol. ix, pp. 545-548, pi. 42, figs. 6 and 7. 

 tU. 8. Geol. Sur. Terr., vol. ix, p. 594, pi. 42, fig. 9. 



