CORRELATION OF SUNDANCE AND MORRISON 259 



is as characteristic of the Oxford Clay as its American equivalent is of 

 the Sundance. 



In regard to the four or five species of plesiosaurs from the Sundance 

 [Megalneusaurus, CimoVmsdurus, Diplosaurus, and Plesiosaurus) {Fan- 

 tosa,urus may be the same animal, and this genus, according to Andrews, 

 is "clearly identical with Murwnosaurus of the Oxford Clay of England^^), 

 Wiliiston^- says: 



"These species all agree in Imviiig suiglo-lieaded cervical ribs, and ln-oad and 

 short epipodlals. From a somewhat careful study of the literature of Eng- 

 lish plesiosaurs, the earliest recorded occurrence of forms with single-headed 

 cervical ribs that I can find is in the Oxford Clay [= Oxfordian of Buckman], 

 as is also the earliest of the short epipodial forms. One species described 

 from the Baptanodon [=: Sundance] beds and referred to CimoUasaurus (to 

 which it prol)ably does not belong) has three epipodial bones, as I am satis- 

 tied from an examination of the typti 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 predomhiant in the 

 Cretaceous, the elongated epipodials utterly disappearing. . . . 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 specialization 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 conclusion to be derived from these opinions is that the marine 

 animals of the Sundance formation are indicative of about the age of the 

 English Oxford Clay near the top of the Oxfordian, or rather Divesian, 

 and the earlier part of the succeeding Argovian. There still remains of 

 the Jurassic, therefore, some of the Argovian and all of the Kimmeridgian 

 and Portlandian, which together may have a thickness in England of 

 1,?)00 feet and on the continent a considerably greater one. The question 

 to be answered is, then. Does the Morrison, which is clearly separated by 

 a break from the Sundance, still fall into this Upper Jurassic time, or is 

 it of earlier Comanchian time, but older than the Washita equivalent 

 (Purgatoire) that overlies it? 



AGE OF THE MORRISON 



General statement. — Let us now see what the fossils that are entombed 

 in the Morrison teach. There have been described about 158 species, as 

 follows (slightly altered from Osborn) :^^ 



^^ S. W. Williston : The HaUopus, Bapta notion, and Allan tosannis bods of Marsh, 

 .lour. Geology, vol, 13, 1905, p. 342, 



13 H. F. Osborn: Close of .Jurassic and opening of Cretaceous time in North America. 

 Bull. Geol. Soc. Am., vol, 26, 1915, p. 299. 



