MOOK, STUDY OF THE MORRISON FORMATION 153 



the beds with the Wealden of Europe, which he considered to Ije Jurassic 

 (1895, 1). In discussing the age of the Wealden he correlated it with 

 the Morrison on the basis of its re]3tilian fauna, and considered it, on this 

 evidence, to be Jurassic. It seems to the present writer that, so far 

 as the reptilian faunas of the Morrison and Wealden were concerned, 

 Marsh was arguing in a circle. Marsh (1896, 7; 1896, 4) quoted Seward 

 as favoring the Jurassic age of the Wealden on the evidence of its fossil 

 plants, and Smith-Woodward as maintaining the Jurassic age of the 

 Wealden on the evidence from the fossil fishes. Marsh also considered 

 the Sauropoda of the Potomac beds to be more primitive than those of 

 the Morrison^ and from this judged the Morrison to' be younger than the 

 Potomac. 



Hatcher (1903, -l) held that the reptilian fauna of the Morrison was 

 closer to that of the middle Jurassic of Europe than to the Wealden. 



Lull (1911, 8), discussing the reptilian fauna of the Arundel forma- 

 tion, said: "The character of these dinosaurs, and of the crocodile as well, 

 correlates the beds wherein tliey are found absolutely with the Morrison 

 (Como) of the West. An accurate comparison with European forma- 

 tions is more difficult, as the faunas have fewer forms in common. 

 Pleuroccelus is reported from the Kimmeridgian as well as from the 

 Wealden, but that from the former horizon may readily have been an- 

 cestral to the Arundel type, although the European material is too frag- 

 mentary to admit of a just comparison. Of tlie other dinosaurs, the 

 affinities seem to be entirely, with Wealden forms, Cozlurus being found 

 therein, while Allosaurus compares in point of size and dentition with 

 the Wealden Megalosauriis. Dryosaurus has its nearest European ally 

 in TlypsUoplhodon. again a Wealden type, and the crocodile, Go7iiopkolis, 

 is reported from the Wealden and its marine equivalent, the Purbeclcian, 

 not from the older Jurassic levels. 



'"The M^eight of this evidence would seem to place this fauna beyond 

 the Jurassic into the beginning of Cretaceous times." 



The most complete comparison between the Morrison and correspond- 

 ing vertebrate faunas has been made by Williston (1905, 4), and the 

 following is extracted from his paper : 



"Cetiosaurus longus Owen is from the Great Oolite, or Middle Jura ; 

 C. ghjmptonensis Phillips, imperfectly known, is from the same horizon; 

 while C. hf'evis Owen, also imperfectly known, is from the Wealden, but 

 is referred by Seeley to Ornithopsis, by Lydekker to Morosaurufi. Orni- 

 thopsis Seeley is from the Wealden; 0. humerocristatus Hulke, from the 

 Kimmeridge. Other uncertain forms are from the Wealden of England. 

 Titanosauriis is referred by Lydekker to probable Upper Greensand. 



