WHITE] INTEODUCTOKY REMARKS. 285 



from the Jurassic rocks, one of the latter belonging to the subgenus 

 Alectryonia. 



It was in the Cretaceous period, as already mentioned, that the oyster 

 family reached its culmination. At that time not only was there a great 

 abundance of individuals, the remains of which are found in almost all 

 the fossiliferons strata of that period, but there were living then a greater 

 number of specific and generic forms than had ever existed before, and 

 greater than have existed since. That is, the Ostreidse not only became 

 generally prevalent and abundant in the marine waters of this part of 

 the earth at that time, but the whole family then reached a greater de- 

 gree of variation as regards specific and generic form and feature than 

 it has ever possessed at any other period. 



Two facts in this connection seem to be not a little remarkable. First, 

 the oyster family, which was an inconspicuous one among the mollusca 

 up to the beginning of the Cretaceous period, then assumed, with ap- 

 parent suddenness, a conspicuous position as regards general distribu- 

 tion, numbers of individuals, and wide differentiation of species and 

 genera. Second, at the close of the Cretaceous period this differentia- 

 tion was suddenly contracted to even narrower limits than it possessed 

 at the beginning of the period, although the family still remained a 

 conspicuous one as regards wide distribution, and numbers of individuals. 



The abundance of oyster-life now existing in North American waters 

 is apparent, when one considers the fact that with the present state of 

 the art of preserving fresh foods, and the facilities for rapid transporta- 

 tion, the oyster has become a common article of food all over our country, 

 inland, as well as upon our coasts. It seems certain, however, that the 

 aggregate of oyster life during the Cretaceous period was much greater 

 than it is now. If there had at that time been human beings in ex- 

 istence to whom molluscan food would have been acceptable, the flesh 

 of Exogyra and Gryphwa would, no doubt, have been as palatable as 

 that of the true oyster. Of the, latter, however, there has been no de- 

 ficiency since at least the middle of Mesozoic time ; and their flesh was, 

 without doubt, in every way identical with that of living oysters. 



Although the Ostreidae, as a family, culminated in the Cretaceous 

 period, different species of the genus Ostrea proper were abundant in 

 Tertiary time ; that is, in the period immediately following the Creta- 

 ceous. Certain of these species also reached a larger size than that of 

 any which aie known to have existed before or since, although some 

 overgrown examples of Ostrea virginica that have been found upon the 

 coast of Maine are reported to be nearly a foot and a half long. 



The so called fresh-water oysters of certain South American and 

 African rivers have already been referred to, and it has been shown 

 that they are not true oysters. In the oyster trade also the terms "salt- 

 water oysters " and "fresh-water oysters "are used. The so-called fresh- 

 water oysters of the dealers are true oysters, but they do not come from 

 waters that are entirely, but only comparativelv fresh. True oysters 



