86 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
as North American prototypal characteristics. These discoveries 
of Cretaceous species have been made in the states of Colorado, 
Utah, Wyoming, South Dakota and Montana; and in the Canadian 
territories of Alberta, Assiniboia and Saskatchewan. In vertical 
range these discoveries extend from the base to the top of the Creta- 
ceous series of formations as it exists in the great region just indi- 
cated. The formations, or groups of strata, are, beginning with the 
lowest, the Dakota, Colorado, including the Bear River beds, Pierre, 
including the Fox-Hills, Judith River and Belly River beds, and 
the Laramie. The Dakota group has furnished comparatively few 
molluscan fossils, and the most that need now be said of it is that 
it is not of marine origin. The Colorado and Pierre formations con- 
sist mainly of unquestionably marine strata, with which the fresh 
water groups alternate. The Laramie is the uppermost formation 
of the Cretaceous series and the character of its molluscan fauna 
gives evidence that it was deposited in a body of water that was in 
part fresh and in part brackish. This formation aso contains plant 
remains which have been referred to the Tertiary; and dinosaurian 
remains which are regarded as of Cretaceous age. I now provision- 
ally refer the formation to the latter age, although its molluscan 
fauna might with propriety be referred to the Tertiary. It is in 
the Laramie strata that the greatest number of species of Unio 
have been found that bear the prototypal features which have been 
frequently referred to. Most of these species were found in a few 
fossiliferous layers of limited extent, each of which was probably 
deposited near the mouth of an inlet and not in the stiller waters of 
the lake. The formation from which each of the species repre- 
sented upon the accompanying plates were obtained is noted upon 
the page of explanations which accompanies each of the plates. 
Besides the species which are referred to in the foregoing para- 
graphs and figured on the accompanying plates, Professor R. P. 
Whitfield has published descriptions and figures of six new species 
of Unio which were discovered in strata of the Laramie Group of 
Montana, and which he has named as follows: Unio e@sopiformis, 
U. verrucosiformis, U. retusoides, U. browni, U. percorrugata, and 
U. postbiplicata. All these fossil species present prototypal char- 
acteristics of the living Mississippi Unione fauna in a marked 
degree. Three of them are so closely like three living species re- 
spectively that Professor Whitfield has given names to the fossil 
*“" Notice of Six New Species of Unios from the Laramie Group,” by R. 
P. Whitfield, Bull. Am. Museum of Nat. Hist., vol. x1x, pp. 483-487, plates 
XX XVIII-XL. 
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