282 REPTILIA 



CHAP. 



families cannot well be changed, and terms like super-families 

 and super-orders are sometimes resorted to by those who do not 

 like to look stern facts in the face. 



The sequence of the groups, although arranged as much as 

 possible in ascending order, is of necessity as unnatural as that of 

 the maps in an atlas. We cannot yet construct a satisfactory 

 phyletic tree of the Eeptiles. The Proreptilia connect them with 

 the Amphibia. Next follow the Prosauria with Splienodon among 

 the Prosauri as the key to most other groups. Then follow 

 the Theromorpha, and it is probable that from various branches 

 of these have arisen the Chelonia, Dinosauria, Crocodilia, and 

 Plesiosauria. The descent of the Ichthyosauria is very pro- 

 blematic. The same applies to the Pterosauria and to the 

 Pythonomorpha, but it is possible that they, together with the 

 Sauria, are connected with the Prosauria. 



With all reserve these hypothetical affinities may be expressed 

 by the following diagram : 



)phidia 

 Ichthyosauria Plesiosauria 



Chelonia Pterosauria 



^Pythonomorpha 



Prosauria 

 Proreptilia 



The eleven sub -classes of the Reptilia present so many 

 important differences that it is not advisable to give here a further 

 general account of their structure. The diagrammatic figures 

 A to T on pp. 280, 281, representing various types of skulls, 

 are intended to explain their chief modifications, all referable to 

 Proreptilian and to certain Theromorphous conditions. One of 

 the most important features is that the mandible, which is always 

 composed of many pieces (cf. Fig. 142, p. 550), is invariably 

 carried by the quadrate bone. Diagrams of the generalised 

 skulls of a Bird and a Mammal have been added for comparison. 



As mentioned on p. 278 the vertebrae of the Eeptilia and 

 those of all other Amniota are gastrocentrous ; that is to say 



