24 THE MOKPHOLOOY OF THE SKULL 



coceplialia, Plesiosauria, Icthyosauria, Pelycosauria, Dinosauria and Pareiasauria. In 

 the Theriodontia and Anomodontia the bone which has been-referred to as the vomer 

 is the true homologue of the mammahan vomer, and this is almost certainly also the 

 case in the Chelonia." He then, following the same line of argument, proceeds to 

 demonstrate that the parasphenoid of the Amphibia is the homologue of the mam- 

 malian vomer. 



In comparing the median section of the skull of the Dimetrodon with that of 

 Lystrosaurus (Ptychognafhus), Fig. 6, it is evident that the separate vomer of the 

 Anomodont skull is absent in the Pelycosauria, but it seems probable that the para- 

 sphenoid plate still attached to the anterior end of the basi-sphenoid can be nothing 

 but the developing vomer, thus furnishing ample proof of the theory of the origin of 

 the mammalian vomer as proposed by Sutton and Broom. 



Broom has already shown (:03") that tbe most primitive of the African forms, 

 ProtcrosHchus of the Therocephalia, has a true median vomer (parasphenoid) correlated 

 with vertical plates rising from the inner edge of the pterygoids exactly as in the 

 Pelycosauria. This median plate is present in the mammals and in the Gomphodontia, 

 it is just as certainly absent in all other reptiles ; it seems safe to predict that when 

 the anatomy of the Theriodonts is known that a complete series connecting the Gom- 

 phodonts with the Therocephalia will be shown to have this median plate. 



The prevomers: The specimen number 1002 is of especial value in preserving 

 the thin median plates of the skull. It clearly shows the presence of paired pre- 

 vomers. The prevomers (Broom : 03') are rather stout rods of bones extending from 

 the middle of the premaxillaries backward and downward in a curve to a point oppo- 

 site the end of the palatine. Their form and relations are shown in Figs. 1 and 

 2, PI. VII, and Fig. 1, PI. IV. The curvature of the lower surface makes a vaulted 

 roof to the mouth in the anterior portion. In about the middle of their course the}'^ 

 are free from the bones on the sides leaving a cavity which forms the posterior nares ; 

 the sides of the prevomers at this point are marked by a prominent rugosity of the 

 edge. Superiorly and posteriorly the prevomers join the vertical pterygoid plates ; 

 superiorly the upper edges diverge and receive between them the united plates, pos- 

 teriorly they shade indefinitely into the plates so that it is impossible to fix the exact 

 limits of the bones. 



The loiver jaw : In specimen 1001 the lower jaws are preserved almost perfectly; 

 the coronoid which was a small splint bone seems to be lost from both sides. The 

 posterior portion of the jaw becomes very high by the development of the posterior 

 bones as vertical plates and from the inner side of this region the articular region pro- 

 jects as an almost sessile process made up of various processes from the angular, suran- 



