322 PALAEONTOLOGY. 



sauria to mammals. It is manifested by the combination of 

 crocodilian, chelonian, and lacertian characters in the Crypto- 

 dontia and Dicynodontia ; and by the combined crocodilian 

 and lacertian characters in the Thecodontia and Sauropterygia. 

 Even the Chelonia of the Purbeck period illustrate the same 

 principle, by the more typical number of modified hsema- 

 pophyses, or abdominal ribs, entering into the composition of 

 their plastron. 



The diagram (fig. 110) gives, in the horizontal spaces, a 

 concise view of the geological relations, or distribution in 

 time, of the several orders of the class Reptilia in the vertical 

 columns. In the first column, the dark mark shews that 

 the ganocephalous group represented by the Archegosaurus 

 began, culminated, and ended in the carboniferous period. The 

 Labyrinthodonts, commencing at the top of the coal series, 

 and culminating in the trias, disappear at the base of the 

 oolitic system. Very significant, on the derivative hypothesis 

 of species, is the commencement of the Iclithyopterygia where 

 the Ldbyrintliodontia terminate. The Thecodonts have but 

 the partial relationship to modern Lacertilia which the Laby- 

 rinthodonts bear to the modern Balrachia. The progress 

 of affinity or transition would seem to be from them to the 

 Dinosauria : the triassic forms of the Sauropterygia diminish 

 the interval between the Protorosaur and the Plesiosaur. 



The absence of Labyrinthodont remains in the permian, and 

 of Ichthyosaurian ones in the wealden, is quite compatible 

 with the conviction that both kinds of Eeptilia were elsewhere 

 existing during the periods of the deposition of those strata. 

 A difference of habit corresponding to the difference of struc- 

 ture explains why the Plesiosaurs might leave their remains 

 in shallow estuary beds of the Wealden, whilst the more 

 powerful swimmers were lording it over the fishes in more 

 open seas. Of the true Batrachia, those retaining the tail 

 appear to have been at their maximum during the upper 



