114: Williston — Dinosaur ian Genus Creosaurus. Marsh 



upper half is much flattened, and the edges are thinned, while 

 proximally it is more trihedral in cross-section, the posterior 

 border rather sharp, the anterior one more rounded and the 

 inner surface here more flattened. Longitudinally the external 

 surface is convex, though less so, or nearly straight, in its 

 middle portion. 



The age of the beds whence these fossils are derived, the 

 Atlantosaurus Beds of Marsh, I have no hesitancy in accept- 

 ing as Lower Cretaceous. They were first referred to the 

 Wealden by Marsh at the time of the discovery of the rich rep- 

 tilian fauna in 1877, but afterwards wrongly placed by him in 

 the Upper Jurassic. I have always doubted this reference, and 

 their Cretaceous age it now seems to me to be sufficiently well 

 proven to accept without question. The character of the 

 reptilian forms present sufficient evidence, I believe, to refer 

 them to beds equivalent to the Wealden of Europe, and the 

 evidence from the invertebrates is still stronger : " The Wealden 

 formation of England contains the greater part of the genera 

 which occur in the Atlantosaurus Beds and is doubtless of the 

 same age. The two formations have similar lithological char- 

 acters, and four of the genera — Unio, Valvata, Planorbis and 

 Viviparus — which are represented in the two formations by 

 species having practically the same development, are not 

 known from older formations."* 



The name Atlantosaurus JBeds, derived from a synonym, is 

 not available for them, and must be replaced by Como Beds as 

 proposed by Scott, unless indeed, the determination is suffi- 

 ciently exact to allow the name Wealden to be substituted. 



* Logan, Kaiis. Univ. Quart., ix, 132, 1900. 

 University of Kansas, Lawrence. 



