18 NON-MARINE FOSSIL MOLLUSCA. 



very favorable for the developement of brackish and fresh- water faunae; 

 while whatever of estuary deposits may have been made during the 

 periods tbat have elapsed since paleozoic time in those regions which 

 now constitute other portions of the continent, have been nearly or 

 quite destroyed by the geological changes that have occurred, or they 

 have hitherto escaped discovery. Consequently, as has already been 

 remarked in relation to the Ostreidse, discussion of not only the brack- 

 ish-water representatives of these three families, but of all brackisk- 

 water forms will cease in this article with the references that are made 

 to the fauna of the Laramie period. Discussion of the fresh-water and 

 land mollusca will be continued to a much later epoch by references to 

 their fossil remains, because favorable and extensive fresh water con- 

 ditions continued in Western North America long after the wide-spread 

 brackish waters of the Laramie period had ceased there. 



But even as regards these -purely fresh- water and land molluscan 

 families, few of their remains have yet been discovered which are refer- 

 able to the epochs which passed between the. Eocene and the present 

 time. These deficiencies of the geological record and their zoological 

 bearing will be made apparent as the different known faunae are pre- 

 sented in their order on the following pages. 



UNIOXIMS. 



For various reasons, no family of non-marine fossil mollusca is of 

 greater interest than the Unionidaa, especially since the discovery of the 

 large number of species in the Mesozoic and Ceuozoic strata of Western 

 North America, and of the rich Unioue fauna of the Tertiary deposits of 

 Eastern Europe. 



Although certain shells found in the Carboniferous and Devonian 

 strata, of Europe and America have been referred to the Unionidae by 

 different authors, the accuracy of such reference has been by others 

 seriously questioned; and American paleontologists at least have of late 

 years not generally recognized as belonging to jthat famliy any shells 

 found in strata of earlier than Mesozoic time.* Prof. James Hall has, 

 however, recently expressed the opinion t that the two bivalve species, 

 which were named by Vanuxem Cypricardites cattsldllensis and 0. angus- 

 tata, respectively,! belong to the genus Anodonta; and that the Mont- 

 rose and Oneonta sandstones (later Devonian) in which those species 

 occur, were "deposited under estuary and fresh-water conditions." 



Aside from the apparent Unione characteristics of these shells, and 

 the other facts upon which Professor Hall bases the opinion he has ex- 

 pressed, the wide differentiation, whichis now known to have become fully 

 established among the Unionidce, at least as early as the later portion of 

 Mesozoic age, points to a very early period for the origin of the family. 



* See remarks on a followiug page ou spurious and doubtful species. 



! See. '• Science " for December, 1SS0. 



jVanuxern's Rep. Geo]., 3d District, New York, p. 186. 



