XV. 2 



CLASSIFICATION 



39i 



years of the Permian period, by which time the main reptilian types 

 had appeared. The individual reptilian orders nearly all became 

 established during the subsequent 45 million years of the Trias and 

 most of them reached their maximum development in the Jurassic 

 and Cretaceous. 



2. Classification of reptiles 



Since our knowledge of reptiles depends mainly on fossil remains 

 it is convenient to classify them by means of the skull into four great 



Psrapsida 



Diapsida 



Fig. 223. Diagrams of reptilian skulls to show arrangement of the temporal openings. 



Anapsida, no opening. Synapsida, a lower opening, with post-orbital and squamosal 



meeting above it. Parapsida, an upper opening with post-orbital and squamosal 



meeting below it. Diapsida, two openings, separated by a bar. 



.jugal; pa. parietal; po. post-orbital; sq. squamosal. (From Romer, Vertebrate Paleontology, 

 Chicago University Press.) 



groups (Fig. 223). Such a classification is in some ways artificial, but 

 it serves to indicate in a broad way the main lines of evolution within 

 the class. 



In the cotylosaurs the dermal bones of the temporal region of the 

 skull presented an unbroken surface and there were no temporal 

 fossae. There were therefore no arches or 'apses' of bone in the tem- 

 poral region. Such forms are placed in the subclass Anapsida. The 

 jaw muscles took origin from the deep surface of the temporal side 

 wall, between it and the brain-case, and they passed down through 

 holes in the palate to be inserted on the lower jaw. This represents the 

 most primitive condition found in reptiles, and resembles that in the 

 early amphibians. It is still seen today, though often in a modified 

 form, in the Chelonia, which are hence placed in the anapsid sub- 

 class. 



In more advanced groups of reptiles fossae bounded by bony 

 arches appear in the temple region, enabling the jaw muscles to extend 



