868 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Judging from tbe characteristics of existing land-locked seas, it is 

 difficult to understand clearly how fresh and brackish waters could have 

 existed iu one and the same sea iu the absence of, or at a distance from, 

 the mouths of tributary rivers ; but the character of the deposits of the 

 Laramie Sea, as well as its molluscan fauna, warrants the suggestion 

 that many comparatively large portions of its area were, at different 

 times and in different places, in the condition of marshes, which were 

 only slightly raised above the general water-level, upon which fresh 

 waters from rains accumulated, and gave congenial habitat to such 

 members of the molluscan fauna of the period as would preferably avoid 

 the brackish waters. This view is supported by the occasional presence 

 of land-shells among those of branchiferous mollusks, the more common 

 occurrence of palustral shells, the occurrence of deciduous leaves, and 

 other fragments of vegetation, all in the same or associated strata j and 

 also the presence of numerous beds of lignite throughout the group. It 

 is also supported by the fact that the fossil Mollusca are found, not uni- 

 formly distributed throughout the group, either vertically or geograph- 

 ically, but to occupy small, distantly separated areas, which are not 

 only locally restricted, but within which locally restricted areas the 

 vertical range of the different species is limited. Admitting that such 

 conditions prevailed, it is easy to understand how it may have happened 

 that certain layers containing the remains of Mollusca^ which could have 

 flourished only in salt or brackish waters, as, for example, Ostrea and 

 Anomia, are found to alternate in close succession with those containing 

 an abundance of fresh-water species, and also with those containing a 

 commingling of types. The conditions thus indicated would have 

 brought the brackish- and fresh -water habitats of those Mollusca into 

 such juxtaposition that they must have frequently encroached upon 

 each other. This frequent encroachment, or mingling of habitats, and, 

 no doubt, the frequent impracticability of retreat, would have had a 

 tendency to inure at least a portion of the mollusks of each to an exist- 

 ence in the other. It is evident that many of the Laramie species were 

 capable of such an interchange of habitat without disadvantage, and 

 that among these were certain species of the Unionidoe, Ceriphasiidw, 

 and allied families. 



In expressing the belief that, with the exceptions referred to, the 

 Laramie Sea was a great body of brackish water, I have not lost sight 

 of the fact that some living mollusks belonging to families that are 

 regarded as of distinctively marine habitat are known to inhabit fresh 

 waters ; nor of the fact that some others which are regarded as of fresh- 

 water types are occasionally found in brackish waters. It seems impos- 

 sible, however, to account for the commingling of types which we find 

 in the Laramie strata, except by assuming that they all lived and 

 thrived together in the same waters, as before stated. 



Before leaving the discussion of the general characteristics of the 

 Laramie Group, the existence in it of a remarkable local or regional mol- 



