236 CALIFORNIA ACADEMY OF SCIENCES. 



back as the "Laramie Group," on the upper borders of the 

 Cretaceous Era, probably older than the Californian Coal- 

 measures, including five forms, while only two species are 

 referred to the miocene epoch, and none of later date. 



I now propose to consider what we know as to recent ex- 

 tinction of species, a subject as yet little investigated, but 

 of great interest in its relations to the geological history and 

 origin of species. Perhaps no class of animal remains is 

 better suited for illustrating the subject of specific descent 

 and transformation than the Land and Fresh-water Shells. 

 Much has already been done in Europe towards elucidating 

 their prehistoric changes, chiefly, perhaps, because there 

 are more extensive tertiary and later deposits containing 

 them, and more excavation of such beds has been made 

 than here.* 



The onh r American observations on the subject, known to 

 me, are those of Prof. C. B. Adams and Thomas Bland on 

 species of the West Indies and Eastern America. (See 

 Binney's Bibliog. of N. A. Conchology for list of these 

 articles.) 



It has been lately observed by Eastern American collec- 

 tors, that many species, once numerous in the older settled 

 districts, have become scarce, on account of the destruction 

 of the forests in which alone most of them can find shelter 

 and subsistence. Some authors have even predicted their 

 complete extermination as cultivation gradually extends 

 over the country. But there must always remain numerous 

 rocky and precipitous tracts, left to the natural tree-growth, 

 which will be safe refuge for land shells. The shores of 

 rivers, also, where trees are less cut away, and annual floods 

 bring down new specimens from their rugged mountain 

 sources, can never be entirely deprived of their indigenous 

 species. The vast numbers found imbedded in the banks 

 of some of the western rivers, and also lying on the surface, 



* See Hyatt in Ingersoll's Report on Molluscs, U. S. G. and G. Survey of the Territories, 

 1876, p. 403. 



