IN THE CARNIVOHUUS 1>1N0CK1'UAL1AN llEPTlLlCS. (i/'O 



Section B, B. — This section is immediately behind the last 

 molar. Here the jaw is still mainly formed by the' dentniy, 

 which forms the whole of the outer side and about half of the 

 inner. The splenial is still a fair-sized element, which forms 

 nearly half of the inner side. The angular is, however, in this 

 section considerably larger than the splenial. 



Section 0, 0. — This section, a little distance behind the previous 

 one, is near the posterior part of the dentary. The largest 

 element in this section is the angular, which forms the main 

 portion of the lower half. On its outer side is a section of the 

 lower posterior j^rocess of the dentaiy. On its inner side is a 

 section of the posterior part of the splenial. The upper part of 

 the section is formed by the coronoid portion of the dentary. 

 It is a powerful and curved structure. Underneath it is the 

 anterior process of the surangular. On the inner side of the 

 section is seen a large part of the preaiticular cut across, and 

 above this is a section of the feebly -developed coronoid element. 



Section 1), D. — This section is through the middle of the 

 posterior opening into the jaw. Above is a section of the 

 surangular capped by the coronoid portion of the dentary. Below 

 is the large angular with articulated to it the prearticular. 



Section E, E. — Between the previous section and this one the 

 angular splits into two plates, an outer and an inner. As seen 

 in this section, the inner plate is closely attached to the surangular, 

 and the outer plate, which is much the thicker, is quite free from 

 the inner both above and below. The inner plate has attached 

 to its lower end the prearticular, and between the two is the 

 posterior part of the opening into the jaw. The surangular is 

 still a powerful bone. 



Section F, F. — This section is only a short distance behind the 

 preceding. The anterior part of the articular is cut across. It 

 is formed of soft spongy bone. Round its lower side lies the 

 prearticular. Above the articular and partly attached to it is 

 the large surangular. Outside the surangular is the inner plate 

 of the angular. Except in its lower part it is quite thin. Above, 

 it broadens out into a delicate bony cap. In the specimen this 

 is somewhat crushed, and is restored in the drawing into what is 

 probably its original condition. Though close to the outer plate 

 it probably is not united to it. The outer plate is very thick. 

 Between the two plates is quite a large cavity. 



In discussing the affinities of this jaw with the jaws of other 

 early reptiles, we are hampered by the fact that in only a few 

 forms is the jaw structure fully known. In fact, as it is only in 

 the last twenty years that the jaw structui'e of living forms such 

 as the Lizard and Tortoise has become fully known, we paleonto- 

 logists may perhaps be forgiven for not having yet been able to 

 X'cveal the structure of the jaw in all our extinct t^-pes. We do, 

 however, know the jaw structure fully in three groups of 

 Therapsids — the Anomodonts, the Goi'gonopsians, and the 

 Oynodonts — and fairly well in the homodont Dinocephalians. 



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