WHITE.] INTRODUCTORY REMARKS. 287 



well as in marine waters. These conclusions are all the more reliable 

 because the other living moUusks referred to are, as a rule, more re- 

 stricted than oysters are, to certain degrees of saltness of tlie waters in 

 which they live. 



Reasoning from such facts as these, it is inferred that the fossil gen- 

 era Exogyra and Gryphaa were denizens of the open-sea; that is, of the 

 numerous species of these genera that have been discovered in the rocks 

 of various parts of the world, none, so far as I am aware, have been 

 found associated with such other fossil forms as indicate a brackish-water 

 habitat, but all their associates indicate that they were denizens of 

 mariue waters.(a) The typical forms of Ostrea, on the contrary, while 

 they occur abundantly in strata of different periods, mingled with mnrine 

 associates, have been found also abundantly associated with other mol- 

 luscan remains that we are compelled to regard as indicating a brackish- 

 water habitat. Therefore we infer that the various species of the genus 

 Ostrea proper have always been capable of living in both marine and 



brackish waters. 



The geology of North America furnishes a most remarkable example 

 of an abundance of brackish-water oysters during one of its geological 

 periods. In that period, now known as the Laramie, and which imme- 

 diately succeeded that in which the uppermost of the marine Cretaceous 

 deposits were made, there existed the most remarkable inland sea that 

 the earth has ever known. Its most southern limit, as at present known, 

 is in Mexico, and its most northern in British America. Its fossil mol- 

 luscan fauna shows that, like the existing Caspian, the waters of that 

 sea were not of marine saltness, but brackish and fresh, or nearly so, in 

 different parts and at different times respectively. Its present known 

 moUuscan fauna was illustrated in the report of the director for last 

 year. Among its moUuscan remains there is an abundance of oyster 

 shells, which are found at isolated localities throughout that great for- 

 mation. The presence of these shells, occurring as they do in many of 

 the layers, shows that the waters in which they were deposited contained 

 at least enough salt to make them brackish. The absence everywhere 

 of true marine forms shows that the Laramie sea was nowhere and at 

 no time of full marine saltness. In the deposits of all that great intra- 

 continental sea no shells of either Exogyra or Gryplim, nor any of the 

 subgenus Alectryonia have been discovered. 



All Che remains of the oyster family which that great formation has 

 yet furnished belong to the genus Ostrea proper. These facts are un- 

 derstood to indicate that the first named generic forms, as already in- 

 timated, were not capable of existing in any waters that were not of full 

 mariue saltness, while Ostrea proper throve abundantly in brackish 



a It is rossible that Ihese genera also entered the estuaries that existed while they 

 flourished, and that a knowledge of the fact has escaped us because estuary deposits 

 of former geological ages are so rarely discovered. It is true, nevertheless, that the 

 OstrcidiB of those genera flourished abundantly in association with mollusks and 

 other animal forms that are characteristic of the open sea. 



