Lacertilia Ophidia | / Ichthyosauria Plesiosauria | / \ \ | / \ \ Crocodilia |/ ? \ | Sauria \ | | Chelonia \| | Pterosauria \ Dinosauria Sphenodon | / \ | | | / \ | | | / Pythonomorpha \ | | |/ | Theromorpha |––––––––––––––– | | 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.