416 THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



and some of them even passed into aquatic types (Mesosaurus, 

 Nothosaurus, Mixosaurus). They are known both in Europe 

 and North America, in India, in South Africa, and in South 

 America; so that their geographical range may almost be 

 regarded as cosmopolitan. Most of the earlier and more 

 generalized forms are closely similar to the Stegocephalia, some 

 apparently derived from the typical labyrinthodonts (Paria- 

 sauria), others more probably connected with the microsaurians 

 (Proterosauria). The later types belong to a higher grade 

 and are more varied. In the Triassic strata of Europe there 

 are remains of many typical Anomodontia, at least one type 

 of ancestral Chelonia (Proganochelys), and ancestral Dinosauria, 

 which might likewise in part be the forerunners of the 

 Crocodilia. There are, in addition, some typical highly- 

 specialized Rhynchocephalia (Rhynchosaurus, Hyperodapedon). 

 Several of these forms are also represented in the Trias of 

 North America and India ; and Anomodontia greatly prepon- 

 derate in the reptilian fauna of the corresponding formations 

 of South Africa. Some small limb-bones (Agriosaurus) from 

 rocks of unknown age on the N.E. coast of Australia are 

 supposed to belong to a Triassic type of dinosaur ; but these 

 are the only indication hitherto discovered of Mesozoic land- 

 reptiles in the Australian region. 



JURASSIC. 



During the Jurassic and Cretaceous periods there were 

 many changes in the distribution of sea and land in the 

 northern hemisphere, and most of the existing continental 

 areas seem to have been subdivided into islands of variable 

 extent. Considerable portions of. the existing land in the 

 southern hemisphere were also submerged at times, and the 

 deposits formed in the seas are very extensive. None of the 

 known marine formations of the Jurassic period, however, 

 represent more than shallow seas, and in these the Actino- 

 pterygian fishes of the sub-order Protospondyli seem to have 

 been the dominant race of vertebrate life. In the same fauna 

 there are numerous Cestracioiit sharks and some primitive 

 skates ; there are also true Chimseroids, some of the latter 



