284 FOSSIL OSTREID^ OF NORTH AMERICA. 



as Uxogyra, and not more than two or three well-defined species are 

 known there. Of these, Gryphwa pitcheri, Morton, which is illustrated on 

 Plate XLIX, may be taken as the type. The beak of the lower valve of 

 Gryphcea is nsually strongly curved upward nearly in the plane of the 

 median line, and as a rule not much deflected either to the right or left. 

 The upper valve is like that of Uxogyra, flat, or even slightly concave, 

 and its beak inconspicuous and not deflected or coiled. 



The American species of Grypliaa are very variable, and it is often 

 difficult to say in what particulars many of the specimens differ from 

 Ostrea proper. 



The common living oyster of the Atlantic coast, Ostrea virginica, fig- 

 ures of different varieties of which are given on Plates LXXIII to 

 LXXXII, may be taken as typical of the genus Ostrea proper, although 

 several of the fossil species are equally typical of that genus, as may be 

 seen by reference, for examjtle, to Plates XXXVI and LX. All the 

 living Ostreidse of North America belong to the genus Ostrea proper; 

 all other genera and subgenera of the family having become extinct. 



The earliest known species of the family, which are of Carboniferous 

 age, belong also to the typical genus. Exogyra and Gryphcea were ap- 

 parently introduced in the Jurassic period, (a) flourished during the Cre- 

 taceous period, when the great oyster family culminated ; and both those 

 genera became extinct with the close of the Cretaceous period. But 

 unlike those more differentiated forms just mentioned, Ostrea proper be- 

 gan its existence before the close of Paleozoic time and has also out- 

 lived both of its kindred but more differentiated genera. The subgenus 

 Alectryonia was introduced with Gryphaa and Exogyra, but it survived 

 them only one geological epoch, when it also became extinct. 



The remains of the earliest known oysters, as has already been stated, 

 were found in Carboniferous rocks. Professor de Koninck described 

 Ostrea nohilissima from the Lower Carboniferous of Belgium, and de 

 Yerneuil described 0. matercula from the Permian of Russia. Prof. A. 

 Winchell described 0. patercula from the Lower Carboniferous of Iowa, 

 but no other trace of the family has been discovered in any other rocks 

 of the Carboniferous age in North America. It is quite clear, however, 

 that the oyster existed through the whole of that age, but it is also quite 

 clear that it was never so abundant in that age as it became in the next, 

 and as it remains to the present day. 



Fossil oysters are not unknown in the Triassic rocks of Europe, but 

 none have yet been found in North American strata of that age. In the 

 Jurassic strata of this continent the family is only feebly represented, 

 at least as compared with those of the Cretaceous period. Only one 

 species of Gryphwa and three species of Ostrea have been published 



a Exogyra is known in European Jurassic strata, but in North America no species of 

 that genus is known in any strata earlier tban those of the Cretaceous period. TLiere- 

 fore in our studies of the North American rocks we regard Exogyra as distinctively 

 characteristic of the Cretaceous period. 



