MEDIAEVAL OR MESOZOIC TIMES. 205 



most entirely Cretaceous, while its lacustrine species are most closely 

 allied to the Tertiary. Oysters [Ostrea) are the most abundant in 

 species and individuals of all the marine forms. 



The following marine mollusks of Cretaceous type are reported 

 from this group; the first three being given by King, and the re- 

 mainder by Meek.* 



Avicula Xebrascana. 

 Nucula cancellata. 

 Ammonites lobatus. 

 Ostrea glabra. 

 Ostrea subtrigonalis. 



The following brackish water species are also given from this 

 group, by Meek: 



Cyrena, one species. 

 Carbicula, five species. 

 Carbula, three species. 

 Certhida, one species. 

 Hydrobia, five species. 

 Micropyrgus, one species. 



The followiug are the fresh water species : 



Unios, four species. 

 Sphaerium, five species. 

 Limnasa, one species. 

 Planorbis, four species. 

 Bullimus, three species. 

 Goniobasis, nine species. 

 Viviparus, eight species. 

 Campeloma, three species. 

 Valvata, three species. 



There are some others inhabiting both brackish and fresh water* 

 A few land shells of the genera, Helix and Hyalina are also found 

 mingled with the above. 



The most remarkable, however, of all the facts connected with 

 the animal remains of this group, is the presence of reptilian re- 

 mains of Cretaceous type. At Black Buttes station, about half-way 

 up the base of the bluff, are laminated light gray shales over a coal 



♦See Meek's Invertebrate Palaeontology of the Cretaceous and Tertiary Fossils of the Upper 

 Missouri, Vol. IX. of U. S. Geological Surveys, F. V. llayden, Geologist. 



