MEDIEVAL OR MESOZOIC TIMES. 177 



tion would be impossible, owing to tbc specific gravity becoming 

 too great to be moved by muscular power. Apparently, to over- 

 come this obstacle, its bones were made partially hollow, similar to 

 those of birds. Atlantosaurus montanus was almost as large as the 

 preceding. Eleven additional reptile forms were found in these lo- 

 calities, some of which were also of gigantic mould. One of them, 

 however, Creosaurus atrax, was a small carniverous Dinosaur. It 

 is also curious that among these gigantic forms there were two of 

 the smallest Dinosaurs yet discovered. One of them was not larger 

 than a cat. Another reptile found here is the type of a new group, 

 and is named by Marsh Stagosauras armatus. A crocodile found 

 here had biconcave vertebrae like a fish. A small animal, structured 

 like a possum (mars?tpial), was also found among these remains. 



As observed already, some of these remains are the most gigantic 

 land animals yet discovered. No land vertebrates approaching 

 them in size have ever been discovered anywhere eke. They 

 probably represent but a tithe of the fauna of that period. As the 

 general slope of the continent at that time was westward, and many 

 great rivers must have flowed from the direction of Nebraska into the 

 old Jurassic sea, it is almost absolutely certain that these gigantic 

 land animals were carried there from the east, and that they repre- 

 sent the fauna of this territoty during the Jurassic period. If, there- 

 fore, we picture to ourselves the climate of that time, its curious 

 forests of tree ferns, conifers, zamias and cycads, full of all sizes of 

 reptilian life, and especially of the gigantic forms, along with a few 

 lonely mamalian species, and some reptilian birds, it will give a 

 faint idea of what Nebraska and much of the adjoining State of 

 Kansas was during the Triassic and Jurassic periods. 



Close of the Jurassic Period. — The Jurassic period was brought 

 to a close by a further contraction of the cooling globe. One of 

 the results of this contraction was, according to Whitney, the up- 

 rising of the Sierras. The rocks of the next period (Cretaceous) 

 lie unconformably on or against its side. At the same time, the 

 Wasatch, almost parallel with the Sierras, and the Uintas, almost 

 at right angles with the last, also came up from the bottom of the 

 old Jurassic sea. This probably raised the whole of this portion of 

 the continent to so high a level as to drain the whole of what had 

 been the Jurassic sea, and constituted it a land surface until the 

 middle Cretaceous period. 

 12 



