342 S. W. WILLISTON 



of Baptanodon with the Jurassic Ophthalmosaurus, the only vertebrate 

 which hitherto has been considered in their correlation. 



I have studied all the types of the described species of plesiosaurs 

 from these beds, and have examined all other material known from 

 this horizon. These species all agree in having single-headed cer- 

 vical ribs, and broad and short epipodials. From a somewhat 

 careful study of the literature of English plesiosaurs, the earliest 

 recorded occurrence of forms with single-headed cervical ribs that I 

 can find is in the Oxford Clay, as is also the earliest of the short epi- 

 podial forms. One species described from the Baptanodon beds and 

 referred to Cimoliasaurus (to which it probably does not belong) 

 has three epipodial bones, as I am satisfied from an examination of 

 the type specimen. The earliest European species having three 

 epipodials, so far as I can ascertain, is from the Kimmeridge. All 

 these characters are specializations, which became predominant in 

 the Cretaceous, the elongated epipodials utterly disappearing. While 

 species with two epipodials continue quite into the Fort Pierre Cre- 

 taceous, the length of the bones is materially lessened. The con- 

 clusion, therefore, to be derived from the plesiosaurs is that the beds 

 are not older than the Kimmeridge. This conclusion is, of course, 

 not decisive, as it may be that such specializations will yet be found 

 in older forms in Europe, and since we can conceive of a more advanced 

 evolution of the plesiosaurs in the western continent during these times. 



The single crocodile described or named from these beds by Marsh 

 presents no trustworthy evidence yet. Marsh referred the species 

 to the genus Diplosaurus, 1 probably identical with the Wealden genus 

 Goniophilis, and originally described from the Atlantosaurus beds. 

 Should it prove to be rightly determined generically, it would point 

 strongly to the Upper Jurassic, since no brevirostral crocodile is 

 known from older rocks. 



ATLANTOSAURUS BEDS 



The age of the Atlantosaurus beds of Marsh, the Morrison beds 

 of Cross, the Beulah shales of Jenney, the Como beds of Scott, 

 has been variously discussed by Marsh, Osborn, Knight, Ward, 



1 American Journal of Science, Vol. L (November, 1895), p. 405. See also Marsh, 

 "On the Geology of the Eastern Uintah Mountains," ibid., Vol. I (March, 1871), 

 Sep. p. 7. 



