ANNOTATED AND IULTJSTRATED CATALOGUE. 



OONCHIFEBA. 



The families of the Conchifera, which are represented by the fossil 

 species discussed in this article, are those only which are represented 

 among the living non-marine mollusca; because, so far as we now know, 

 there is not a single representative of an extinct family among all those 

 species. Moreover, the principal genera are the same among both the 

 fossil and recent forms, and in only a few cases at most is there a sub- 

 generic difference, or even a difference in the subordinate types into 

 which certain of the genera may be divided; although all the species 

 are regarded as extinct. 



It should be understood that this article is a review, and* not a re- 

 vision, of published species. Therefore, the genuineness of the different 

 species which have -been published by various authors is seldom called 

 in question, even when it is "doubted, as it is in a number of instances. 

 It is thought best on this occasion to present the subject somewhat his- 

 torically, and defer a critical revision of the species to another time. 



OSTREID.E. 



Although the Ostreidse of the present time are much less frequently 

 found living in the waters of the open sea than in those of bays and 

 estuaries, the fossil shells of all the various generic groups of that 

 family are often found quite abundantly associated with those of such 

 molluscan forms as must be regarded as having been denizens of the 

 open sea. Indeed, both the Mesozoie genera Gryphcea and Exogyra seem 

 to have been invariably of open-sea habitat; while Ostrea proper has, 

 in all the Mesozoie and Cenozoic epochs, existed in both marine a'nd 

 brackish waters; but the last-named genus seems to have always been 

 in past times the only representative of the OstreidaB that has lived in 

 brackish waters, while none of the family have probably ever lived in 

 perfectly fresh waters. 



The most noteworthy examples of the fossil brackish-water Ostreida3 

 of North America, namely, those of the Laramie Group, occur in strata 

 that contain no truly marine forms, but which are frequently found to 

 closely alternate with other strata in which fresh-water and land mol- 

 lusca prevail; and, indeed, there are often found associated with these 

 shells of Ostrea those of species whose liviug representatives exist only 

 in fresh waters. But as it is my intention to prepare a separate essay 

 for a future report on all the fossil OstreidaB of North America, little 

 more than incidental reference will be made to this family in this arti- 

 cle even to those species of Ostrea which constitute parts of the brack- 

 ish-water fauna herein discussed. Since, however, the molluscan fauna 



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