ON THE TEMPORAL ARCHES OF THE REPTILIA. 17 



2, On the Temporal Arches of the Reptilia. 

 By n. Broom, D.Sc, F.R.S., C.M.Z.S. 



[Received November 15, 1921 : Read Felivuary 31, 1922.] 



(Text-figures 1-4.) 



In the last thirty years most of the attempted classifications of 

 the Heptilia have been based on the structure of the temporal 

 arches. It is generally agreed that all the later reiDtiles 

 have descended from a Lower Permian ancestor or ancestors 

 that had a roofed temporal region, and the arches found in 

 later forms have manifestly resulted from a fenestration of the 

 ancestral roof. 



Many reptiles have a temporal region which has resulted 

 from one opening in the ancestral roof : others have a temporal 

 region in which two openings have been formed. If all later 

 reptiles could be classed into those with two tempoi'al openings, 

 those with only one, and those with none, and if we could be 

 quite certain that the pair of openings in those reptiles with two 

 and the single opening in those Avith one were homologous the 

 classification would be a very easy matter. Unfortunately, we 

 have a number of groups where thenatui'e of the temporal region 

 is not quite clear, and at present there is no general agreement 

 as to the origin or affinity of these groups. 



There is almost universal agreement that at least all the later 

 reptiles with two temporal openings are closely related and form 

 a natural group to which the name Diapsida has been given by 

 Osborn. In this group we have the Dinosaurs, Crocodiles, 

 Phytosaurs, Pterosaurs, Rhynchosaurs, Rhynchocephalians, and 

 the primitive order I added a few years ago, the Eosuchia. 

 There is also general agreement that the Mammal-like reptiles 

 with a single temporal opening form a second natural group. 

 But we are left with the following important orders, concerning 

 which there is doubt — Ichthj^osauria, Plesiosauria, Placodontia, 

 Chelonia, and Lacertilia. 



The Ichthyosaurs, Plesiosaurs, and Placodonts agree in having 

 a single temporal opening. The doubt is whether this single 

 0]>ening is homologous with the single opening in the temporal 

 region of the Mammal-like reptiles. The very large majority of 

 morphologists are of opinion that it is — I think it is not. 



The Chelonians have a temporal region which looks as if it 

 had never been fenestrated at all, but merely in many types 

 encroached on from behind or below. Some hold that the 

 temporal region is still a simple roofed one. But there are 

 at least two other possibilities. The Chelonians may be derived 

 from an ancestor which had one temporal opening like the 

 Plesiosaur, or two as in Sphenodon,, Avhere the openings had 

 become secondarily closed, or they may have originated from an 



Prog. Zool. Soc.--1922, l^o. II. 2 



