70 Prof. Owen on the Reptilian Fossils of South Africa. 



in thickness and forming an angular groove on its inner side, as shown in the sec- 

 tion through the posterior third of the ramus at 26, fig. 5. PI. IV., joining the splenial 

 element 25 a by its lower inwardly inflected border. At the middle of the ramus 

 it presents an angular excavation on its outer part, pointed behind for the reception 

 of the lower pointed branch of the dentary piece, and increases in depth as it di- 

 minishes in thickness, forming the thin plate on the inner side of the dentary piece, 

 exposed in the middle section, fig. 6, 26 ; the lower border of the angular there 

 curving inwards and forming a groove for the reception of the lower border of the 

 splenial element 25 a, which presents two grooves for the reciprocal reception of 

 the edges of the single groove in the angular piece, the two elements being there 

 doubly interlocked together. The dentary piece, which forms the whole of the strong 

 and large simple symphysis, commences in each ramus behind by a bifurcate ex- 

 tremity, the upper fork reaching to the posterior third of the ramus : it is shown 

 at 24 in the section through that part. It there joins the sur-angular (25), and 

 increasing in thickness as it advances forward soon joins the coronoid, and now 

 takes on a trihedral form as in fig. 6, 24, the outer angle forming the strong outer 

 ridge before described : it also increases in depth, and at the last section the upper 

 and lower forks have united ; the lower fork forming the thin plate of bone applied 

 to the outer side of the angular piece, and fitting into the angular notch in that bone. 

 About half an inch anterior to the bifurcation, the portions of the dentary in each 

 ramus unite to form the symphysis, which has already been described, and which 

 showsno trace of median suture. A considerable vacuity occupies the substance of 

 each ramus, bounded at the middle of the ramus, as shown in section fig.. 6, by the 

 dentary and angular pieces externally, by the splenial and coronoid pieces inter- 

 nally, by the sur-angular and coronoid above, and the interlocked opercular and 

 angular pieces below. This vacuity communicates with the external surface in the 

 form of a narrow elongate fissure, bounded anteriorly by the two forks of the dentary, 

 and behind by the upper fork of the dentary, the sur-angular and the angular 

 pieces. This fissure appears to have been closed on the inner side of the ramus by 

 the splenial or opercular piece. 



The alveolar border of the lower jaw is slightly grooved, as in the Trionyx ; the 

 outer border of the groove being most produced, and forming the trenchant edge. 

 In the edentulous character of the alveolar border, in the anchylosed symphysis, 

 in the depth of the rami and its general form, the lower jaw most resembles that 

 of the Chelonian Reptiles. In the external fissure above described, and the pos- 

 terior bifurcation of the dentary piece, it resembles the lower jaw of the Crocodile ; 

 but in the more essential characters of its composition it agrees more closely 

 with the Lacertian type of the same bone. Thus the angle of the jaw is formed 

 by the articular, not by the angular piece, as in the Crocodile : the coronoid or 



