332 R. S. LULL SAUROPODA AND STEGOSAURIA OP THE MORRISON 



graphic xalue, as the only genera which may possibly be common to both 

 faunas are comparativeh' generalized ty])es, neither of which is confined 

 to a single geologic level in Europe nor, in the case of Morosaurus, in 

 North America. 



Stegosauria. — The European Stegosaurs include at least two phyla; 

 of these the one represented by the small spinescent Polacanthus of the 

 Wealden has no known Morrison equivalent, the group not appearing in 

 America until the Lakota, possibly still later in time. Of another phy- 

 lum, which included the American genus Stegosaurus itself, there seem 

 to be Old World representatives in Omosaurus durobrivensis, of which 

 the type specimen preserved in the British Museum is from Lhe Kim- 

 meridgian and shows great similarity to Stegosaurus in the character of 

 the veitebra? and in the presence of the expanded armor plates, the chief 

 distinction being the appearance on the femur of a well defined, crest- 

 like fourth trochanter in durohriveynsis which is obsolete in the several 

 American species. A second specimen in the Woodwardian Museum at 

 Cajnbridge, although labeled Omosaurus leedsi by Seeley, has also been 

 referred to 0. durobrivensis by A. Smith Woodward. The two specimens 

 are very similar, and associated with that of Cambridge I saw a very 

 characteristic caudal spine in addition to the armor plates. This latter 

 species is from tlie Oxford clay of Felton, and therefore, if the level be 

 correctly stated, froiii the Michlle Jurassic, in neither of these Old 

 World forms ha\e the xertebrte reached the degree of specialization found 

 in the Morrison types, and T should consider the reduction of the femoral 

 trochanter in tlie latter an cNolutionary advance as well; for, as Dollo has 

 shown, its ])resence is correlated with a bipedal gait which was secondarily 

 lost ill the Stegosaurs. 



The evidence from tlie European Stegosaurs, therefore, can not be 

 taken as indicative of correlated age, Init, as I read it, simply points to a 

 greater anti(niity on tlie ]tart of Oxfordiaii nnd Kimmeridgian members 

 of the grou}). 



MORRISON 



Tlie American Moi'rison contaijis the greatest profusion of Sauropod 

 dinosaurs which any formation or locality has produced. Of the five 

 families recorded, one, the Atlantosaurid*, including perhaps the best 

 known members of the suborder — AtJaidomurus, Apatosaurus, and 

 Jlrontosaurus — is exclusively American. Nearest this family stand the 

 ]\rorosaurid;e, smaller forms the stature of whose greatest member is but 

 two-thirds that of Bronlosaurus. The genus Morosaurus includes five or 

 six American species, of which all but one, which has been identified 



