ART. XXIX.-PALEONTOLOGICAL PAPERS M. 7: ON THE DISTRI- 

 . BUTION OF MOLLUSCAN SPECIES IN THE LARAMIE GROUP. 



By 0. A. White, M. D. 



The term Laramie Group is here used to include all the strata between 

 the Fox Hills Group of the Cretaceous period beneath, and the Wasatch 

 Group ( = Vermilion Creek Group of King) of the Tertiary period above. 

 That is, it includes, as either subordinate groups or regional divisions, 

 both the Judith Eiver and Fort Union series of the Upper Missouri Eiver ; 

 the Lignitic series east of the Eocky Mountains in Colorado ; the Bitter 

 Creek series of Southern Wyoming and the adjacent parts of Colorada; 

 and also the "Bear Eiver Estuary Beds", together with the Evanston 

 Coal series, of the Valley of Bear Eiver and adjacent parts of Utah. 

 Strata of this great Laramie Group are known to exist in other large 

 and widely separated districts of the western portion of the national 

 domain, but only those above indicated are especially noticed in this 

 paper. 



So far as the brackish-water mollusca of the Laramie Group have yet 

 been investigated, they have proved, with few exceptions, to belong to 

 types represented by living moliusks of similar habitat; and the fresh- 

 water and land moliusks of that group of strata belong almost wholly, 

 if not entirely, to types that are fully represented by living species. 

 Therefore a mere similarity or even identity of molluscan types in the 

 strata of the different regions just enumerated would not prove them to 

 belong to the same epoch ; but it is held that an identity of species does 

 constitute such proof. 



During the season of 1877 it was my good fortune to make considera- 

 ble collections of fossils from all the forenamed regions except those of 

 the Upper Missouri Eiver. Study and comparison of my own collections 

 with those made many years ago by Dr. Hayden from the Judith Eiver 

 and Fort Union beds in the Upper Missouri Eiver region shows an inti- 

 mate relationship to exist between the molluscan fauna of each of these 

 series respectively. This fact is illustrated to some extent by the fol- 

 lowing table, which, however, includes only the species that have been 

 discovered in the strata of more than one of the regions, or of the subor- 

 Bull. iv. Ko. 3 12 721 



