JURASSIC PERIOD. 535 



tainty known to have existed. The Plesiosaurs are represented 

 by species of Plesiosaurus itself, and by the allied genera Sitno- 

 saurus, Nothosaurus, Pistosaurus, &c. The Anomodontia are 

 represented by the genera Dicynodon and Oudenodon ; whilst 

 Rhynchosaurus is referred by Owen to this group. Lastly, the 

 Deinosaurs are represented, according to Huxley, by several 

 forms, amongst which the most important are the "Thecodont" 

 Palceosaurus and Thecodontosaurus, both of which have been 

 referred by other writers to different groups of the Reptilia. 



The existence of Birds during the Triassic period must, as 

 yet, be regarded as uncertain. The only evidence as to their 

 existence which has been hitherto obtained, consists in the 

 paired footprints which have been already spoken of as occur- 

 ring in the Triassic strata of the Connecticut Valley. These 

 footprints are very numerous, and are often of very large size ; 

 and there is no doubt but that many of them were produced 

 by animals walking upon two legs. Some of them, however, 

 have been unquestionably produced by Reptiles ; and it must 

 at present remain uncertain whether all have been thus formed, 

 or whether some may not have been formed by Birds. The 

 probabilities, however, are in favour of the view that some of 

 these tracks are truly ornithic. 



Lastly, the Mammals are represented in the Trias only by 

 the small forms referred to the genera Microlestes and Droma- 

 therium, both of which are probably referable to the order of 

 the Marsupials. 



CHAPTER LII. 



JURASSIC PERIOD. 

 ROCKS OF THE PERIOD. 



SUCCEEDING to the Trias, we have a great series of Rocks 

 which are known as the Oolitic Rocks, from their commonly 

 containing oolitic limestones, or as the Jurassic Series, from 

 their being largely developed in the mountain -range of the 

 Jura, on the western borders of Switzerland. The Jurassic 

 rocks are very extensively developed in Britain, where they 

 consist of the following members in ascending order : 



I. Lias. 



II. Lower Oolites (consisting of the Inferior Oolite, Fuller's 



Earth, Great Oolite, Stonesfield Slate, &c.) 



