258 REPORT UNITED STATES GEOLOGICAL SURVEY. 



saltness in the waters. In one layer TJnio and Corbicula in abundance, 

 the former represented by half a dozen species, are associated together, 

 a very large proportion of all of them having their valves together in 

 natural position, showing that none of them had been drifted ; and with 

 these, in the same layer, are associated Neritina and Melania, which also 

 show no evidence of having been drifted. At the same locality there are 

 certain layers, alternating with other fossiliferons layers, which contain 

 Ostrea and Anomia alone, and which probably represent the maximum 

 saltness of the waters that prevailed there. There are also other alter- 

 nating layers, which contain fresh- water types alone, which probably 

 represent the minimum saltness, or perhaps entire freshness of the water 

 that prevailed at that particular place at certain times, and the layers 

 containing a mixture of types probably represent intermediate grades 

 of saltness of those waters. It is remarkable that, with all this varia- 

 tion of their fossd contents, none of the strata present any evidence of 

 littoral or estuary deposition. 



While it seems evident that at different times in certain places these 

 Laramie waters alternated from a decidedly salt to a nearly or wholly 

 fresh condition, it seems equally evident that certain species belonging 

 to different types, the representatives of which are now found only in 

 fresh waters, were then capable of living and thriving in waters that 

 contained a considerable degree of saltness. The species referred to 

 belong to the Unionidce, Cariphasiidw, and Melaniidw, the fact of the asso- 

 ciation of certain species belonging to the first and second of these fam- 

 ines with brackish-water forms at Bear Eiver Valley having been 

 already stated ; and on previous pages the association of Melania wyo- 

 mingensis and M. insculpta with Ostrea and Anomia has already been 

 noted. 



It is a remarkable fact that the species belonging to the three families 

 named, which are found with the brackish-water associates, almost with- 

 out exception present a greater degree of differentiation than those do 

 which are found in later but purely fresh-water deposits ; and also in 

 some cases greater than that which is shown by recent congeneric forms. 

 This fact led me in a former publication* to suggest that the peculiar 

 differentiation that has been attained by our North American Unionidm 

 began under the influence of a certain degree of saltness of the waters 

 in which they lived. 



There are many well-known instances of living species of mollusks, 

 belonging to families that are regarded as of distinctively marine habitat, 

 which are found far up from the mouths of certain rivers, inhabiting 

 waters that are wholly and always fresh, to which habitats they seem 

 to have made their way against opposing and, at first, uncongenial con- 

 ditions. On the other hand it is not to be denied that instances of living 

 mollusks of fresh- water types encroaching upon marine waters are rare; t 



* See Bull. U. S. Geol. and Geog. Sur. Terr., vol. iii, p. 623 et seq. 



t The Baltic, Black, and other tideless seas appear to afford the majority of the known 

 instances of the commingling of living fresh-water with brackish or marine forms, 

 and these occur in estuaries whither the fresh-water forms had been carried from their 

 fluviatile habitats by floods or the ever-present pressure of the river-flow. Fresh-water 

 mollusks in saline waters are not, however, always there by compulsion, because upon 

 the shores of Great Salt Lake, as noted by Mr. Gilbert and myself, a species of Physa 

 and one of Limncea, both of which are common in the fresh waters of that region, have 

 been found inhabiting pools of water that was found to be much too salt to drink ; and 

 at the Hot Sulphur Springs in Middle Park, Colorado, I found the same species in 

 water strongly charged with sulphur. In both these cases, however, the adult size of 

 the individuals was considerably less than that of those found in fresh waters. The 

 presence of tides, even in waters that are always fresh, seems to be quite uncongenial 

 to most if not all species of fresh-water mollusks, and it is probably this condition 

 that aids in preventing the commingling of fresh and brackish water forms. 



