Miscellaneous. 251 



later than the Eocene. All the molluscan remains which have been 

 found in these later deposits belong to familiar living types, although 

 of extinct species. 



That the palustral and land pulmonates might have been, and 

 perhaps were, preserved under immediate conditions differing from 

 those which ensured the survival of the Unionidse is evident ; but 

 certain facts point to the conclusion that the peculiar " North- 

 American " types of Uniones which prevailed in the Laramie epoch 

 were not transmitted through the Eocene, Miocene, and Pliocene 

 epochs as denizens of the freshwater lakes which succeeded the 

 brackish water of the Laramie sea, and each other, in their occu- 

 pancy of a great part of the interior region of North America, up to 

 at least near the close of the Pliocene epoch. The Eocene fresh- 

 water deposits contain a considerable number of species of Unio, it 

 is true ; but they are all, so far as known, of a smooth surface and 

 oval form, and constitute a type which, although common among 

 living Uniones, is exceedingly rare, if not entirely wanting, in the 

 Laramie group. The conclusion therefore seems necessary that those 

 peculiar and varied forms of Unio which have been mentioned in 

 the preceding list, with their faunal molluscan associates, escaped 

 from the Laramie lacustrine waters before the close of that epoch, 

 into those fluviatile waters which form the outlet to the lacustrine, 

 and which became a part of the Mississippi drainage-system as the 

 elevation of the continent progressed*. 



The magnitude of the physical changes which have taken place upon 

 the North-American continent since the epochs in which the Mollusca 

 lived, which are discussed in this article, has already been referred 

 to. These changes were no less than the gradual desiccation of the 

 region formerly occupied by great inland lakes, which for magnitude 

 have now no equals upon the earth, the elevation of the whole 

 Rocky-Mountain system, and the establishment of the present great 

 interior river-systems. Through all these changes these molluscan 

 types have come down to us in unbroken lines, some of which, to 

 speak figuratively, were of remarkable tenuity. It is true there 

 has been a dropping out of some of the earlier associated types and 

 an introduction of new ones as the epochs passed ; but the lines of 

 descent of the numerous types which have reached us unbroken 

 seem to be almost parallel, so little have they changed with the 

 lapse of time. So slightly divergent are these lines, considered as 

 lines of differentiation, that, if we bound them all by two imaginary 

 straight lines, we shall have an evolutional parallax that would 

 carry back the origin of these types to a period inconceivably re- 

 mote. We must therefore conclude that their origin was, at least 

 in some degree, saltatory ; but the real conditions under which they 

 originated must probably always remain obscure. I have, however, 

 elsewheref suggested that the differentiation of the Unionidse took 

 place under the influence of salt in the water in which they lived ; 

 but it is plain that this explanation will not apply to the case of 

 the palustral and land Mollusca. — Amer. Joum. Sci., July 1880, 

 pp. 44-49. 



* This subject is discussed at some length in Bull. U.S. Geol. Surv. 

 Terr. vol. iii. p. 615. 



f Bull. U. S. Geol. Surv. Terr. vol. iii. p. 623. 



