622 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



having the beaks projecting beyond the front. These last-named types 

 are not confined to the strata of any particular epoch, but are found in 

 those of Jurassic, Cretaceous, Post-Cretaceous, and Tertiary age. 



It is these species of the Unionidce, among other fossil species of fresh- 

 water and land Mollusks, that were referred to in the early part of this 

 paper as the ancestral representatives of similar Mollusks now living in 

 North America. Admitting this relationship by descent, one naturally 

 inquires how the necessary conditions for a continuous progenital line 

 could have been preserved from Jurassic, and doubtless much earlier, 

 time to the present. As before remarked, this history, from the very 

 nature of the case, can never be accurately known ; but the following 

 suggestions are offered concerning the assumed relationship of the fossil 

 with the living Unionidw of North America. 



All the fossil Unionidce of Western iSTorth America, so far as is uo'^v 

 known, have been obtained from lacustrine strata, with one or two pos- 

 sible exceptions of what may have been estuary deposits, no evidence 

 having been observed that any o'f those deposits are of fluviatile origin. 

 These lacustrine formations are of very great extent in Western North 

 America, and, without doubt, the lakes in which they were deposited 

 were caused by encircling bands of rising land during the elevation of 

 the continent. These great land-locked waters were at first brackish, 

 but finally became, and for a long time remained, fresh, continuing so 

 until their final desiccation. 



That many species of the Unionidce of the W^est not only originated, 

 but became greatly differentiated, at earlier periods than those in the 

 strata of which the oldest known species have been discovered seems 

 quite certain, because those earliest species are found to be so greatly 

 differentiated as to indicate the lapse of much time in the process, and 

 it is interesting to note that among the results of this early differentia- 

 tion was the production of some of the characteristics which we now 

 recognize as peculiar to North American types. These earliest known 

 American species may have been developed in either lacustrine or fluvia- 

 tile waters, or both, but it seems at least very probable that a large pro- 

 portion of the fossil species that are related to living ones, especially 

 the more differentiated forms, originated in the great lakes, in which 

 the extensive Post-Cretaceous and Tertiary deposits were made, and in 

 loaters that were at least a little salt. That they did not originate by im- 

 mediate evolution from marine species, which must necessarily have 

 become landlocked when the great lakes were first formed, is suggested 

 by the fact that other invertebrate species, such as would doubtless 

 have survived with the Uniones by evolution into other species and gen- 

 era, have not been discovered. 



No trace, for example, of the genus Dreissena, various species of which 

 are so common, both fossil and living, in the Cld World, has yet been 

 found in American strata, although its near relatives, Mytilus and Modi, 

 ola, are not uncommon in the immediately underlying marine Cretaceous 

 strata. The purely fresh-water fauna of those western deposits seems 



