LARAMIE FORMATION. 77 



were laid down in succession and .-it mosl varied topographical horizons 

 the overlying formations — at one point, the Arapahoe; al another, the 

 Denver; at a third, the Monument Creek; and finally the Pleistocene. 

 Each rests n<>t only on the next younger formation but also in places on 

 the Laramie itself. 



LIFE. 

 Animal remains. TllCSC, SO fai' as lit present kllOWll, JITC limited to tWO 



species of mollusks and one of a vertebrate. 



The invertebrate remains include the very characteristic Laramie form, 

 Ostrea glabra, and a Unio sp.?, which arc considered decisive as to the age 

 of the formation The occurrence of Ostrea glabra is general for the field, 

 and always at the same horizon, a short distance above the basal sandstones 

 of the formation. Two Unios only were found, these occurring in different 

 localities and well up in the shaly portion of the formation. 



The vertebrate, according to Professor Marsh, belongs to the order 

 Ornithopoda of the subclass Dinosauria The genus is undetermined. The 

 specimen was found by a ranchman about :><> feet below the surface, in a 

 well sunk through the upper Laramie strata, on the slopes of Dry ('reek, 

 about 8 miles west-southwest of the town of Brighton. 



piam remains. — The ] >lant rema ins in the Laramie are abundant and show 

 a clear differentiation from those of the Arapahoe and Denver beds. Prof. 

 F. H. Knowlton, of the Survey, has made an exhaustive study of the com- 

 bined floras, and his results are embodied in Chapter VII of this report. 



