MOOK, HTUDY OF THE MORRISON FORMATION 155 



family !' Perhaps this opinion is not decisive, but Hypsilophodon cer- 

 tainly cannot be called the most specialized. Lydekker even refers cer- 

 tain Kimmeridge and Wealden species to the American genus Campto- 

 saurtis. 



"Perhaps the best evidence we have for the Jurassic age of the Ameri- 

 can deposits is that of Stegosaurus, which is so closely allied to Omo- 

 saurus Owen from the Kimmeridge that Marsh believed the two genera 

 to be identical. On the other hand, this type of the predentate dinosaurs 

 seems to range from the Lower Lias in Scelidosaurus to Paleoscincus 

 from the Laramie, with four or five genera referred to the group from 

 the Wealden. Its value, then, is slight. 



"Other evidence offered by the reptiles from the American beds is 

 slight. A genus of crocodiles called by Marsh Diplosawrus seems to 

 include Hyposaurus vehhii Cope from the Comanche Cretaceous of 

 Kansas. Years ago Zittel referred both of these forms to the genus 

 Goniopholis from evidence communicated by Professor Marsh, and 

 Goniopholis is said to be 'a genus very characteristic of the Wealden' 

 (Lydekker). The recently published figure of the type specimen of 

 Diplosauriis, when compared with figures of Goniopholis, shows a start- 

 ling resemblance. Indeed, so far as I can learn, there are no brevirostral 

 crocodiles known from below the Purbeck or lithographic slates. The 

 evidence, then, of the crocodiles is decidedly for the uppermost Jurassic 

 or Wealden age of the American beds. 



"Of the Chelonia the single species Compsemys plicatulus Cope 

 (Gluiops ornatus Marsh) is not at all decisive. If the species is correctly 

 referred to Compsemys, all its related forms are of Cretaceous age. 

 Nor is there any evidence to be obtained from the pterosaurs or birds. 

 Of the mammals I will not venture to speak, save that I think that there 

 are too few forms known from the Wealden to offer any basis of compari- 

 son. Of the fishes a few species of Ceratodus only are Icliown, and inas- 

 much as this genus is supposed to range from the Trias to the present 

 time, these species have no correlating value whatever. 



"To sum up : there is no valid vertebrate evidence pointing to an age 

 greater than the Purbeck for the Atlantosaurus beds, and but very little 

 for a greater age than that of the Wealden. 



Unfortunately, in most of the discussions hitherto the Atlantosaurus 

 beds have been considered as some brief epoch. The faunas of the upper 

 and lower parts have never been differentiated, save in some exceptional 

 cases. Marsh, indeed, rarely ever gave any precise location for his type 

 specimens, referring them simply to Wyoming, Colorado, etc. The term 

 'Upper Jurassic' has been applied indiscriminately to the whole fauna. 



