white.] ANNOTATED CATALOGUE. 15 



of the great inland brackish-water sea which existed during the Lara- 

 mie period will be necessarily somewhat frequently referred to and 

 briefly discussed on following pages, it is thought best to make the 

 series of its illustrations the more complete by presenting a few figures 

 of the more characteristic forms of its Ostreidse, which will be found on 

 Plates 9, 10, 11, and 12. 



Five species of Ostrea* have been described and published by various 

 authors from strata which are now regarded as belonging to the Laramie 

 Group; but in view of the known wide range of variation among the 

 species of this genus, and the actual discovery of mauy intermediate 

 forms which connect at least a part of those supposed species together, 

 it is now thought that the strictly specific forms of Ostrea which have 

 been discovered in the Laramie Group do not number more than two or 

 three at most, t 



It is interesting to note how closely some of these ancient species of 

 Ostrea are allied to living forms, a good example of which is afforded by 

 0. wyomingensis, as may be seen by comparing the figures of it on Plates 

 10, 11, and 12, with the shells of the common Ostrea virginica, now liv- 

 ing so abundantly upon our Atlantic coast. So closely, indeed, are some 

 of the fossil specimens like liviug ones that, but for their partially min- 

 eralized condition, the former might easily be taken for damaged ex- 

 amples of the living species. 



While the Ostreidae have formed a more or less prominent feature of 

 all the molluscan faunae whose remains are found in all the marine de- 

 posits from the Jurassic period to the present time, we have yet discov- 

 ered no remains of the family in any North American strata of any of 

 the epochs between the close of the Laramie period and the beginning 

 of the Post Tertiary which can be properly referred to a brackish water 

 origin. Therefore the consideration of this family as contributing any 

 of its species to non-marine molluscan fauna? must cease in this article 

 with the references that are made to the fauna of the Laramie Group. 



Precisely similar remarks may be made concerning the genera Anomia, 

 GorMcula, Corbula, and Neritina so far as regards the extinction of all 

 the species of those genera in the waters of the Laramie Sea as a conse- 

 quence of their becoming completely freshened at the close of that 

 period ; and, also, because of the non-discovery of any brackish water 

 deposits of a later date than that period in which such remains may 

 have been deposited. 



AlTOMIID^E. 



Since among fossil faunas Anomia is an almost constant associate and 

 sometimes, as, for example, in many of the layers of the Laramie Group, 



* Those were named, respectively, Ostrea sublrigonalis Evans & Shuinard ; 0. glabra 

 Meek & Hayden ; 0. arcuatilis Meek ; 0. insecuris White ; and 0. wyomingensis Meek. 

 Examples of all these forms are figured on the plates accompanying 1 ' this article. 



tSee remarks on this subject in An. Eep. U. S. Geol. Sur. Terr, for 1877, p. 162. 

 Also, ib. for 1878, Part I, p. 56. 



