1904,] OF THE THERIODOXT MANDIBLE. 493 



to be somewhat displaced inwards and forwards. On the right 

 side it is displaced veiy considerably, and lies a little in front of 

 the articular. As preserved, it is a moderately flat little bone 

 which probably lay on the front of the inner part of the squa- 

 mosal. It has probably formed the greater part of the articulation 

 for the articular, though at its outer corner the articular not 

 improbably also formed a small articulation with the squamosal. 



Both mandibles show the presence of a supporting plate of bone 

 lying against the inner side of the anterior part of the base of the 

 coronoid process. This is the bone which Seeley believes to be 

 the " coronoid," but which I believe to be the surangular. It is 

 present in all the Theriodont jaws in which this region is exposed, 

 though it is not so well seen in any of the Albany Museum 

 specimens as in some of those in London. 



The type skull of Trirachodon kannemeyeri shows the articu- 

 lations of both jaws fairly satisfactorily (3). On each side the 

 articular and angvilar bones are seen to be almost exactly similar 

 to those in Gomphognathus, though the back part of the dentary 

 is relatively more largely developed in Trirachodon. The splenial 

 is seen to lie on the inner side of the dentary from the symphysis 

 to near the middle of the jaw. 



The quadrate, though not very perfectly displayed, is apparently 

 fixed to the squamosal behind in the same way as in Cynognathus, 

 and though the articular is fairly broad, the quadrate appears to 

 form the whole of the articulation. 



The bone which I regard as the tympanic is present as a delicate 

 bony rod of about the thickness of a pin. It exactly corresponds 

 in situation with the tympanic in Cynognathus. 



A rather badly weathered skull of Trirachodon kannemeyeri, 

 also in the Albany Museum collection, shows a tangential section 

 through the articulation. This is represented in fig. 11 , PI . XXXY., 

 and though the bones are slightly displaced, a good idea is obtained 

 of the way in which the plate-like quadrate lies in front of the 

 squanlosal. 



Another specimen of Trirachodon shows the fractured sections 

 of the mandible. These are shown in figures 7 and 8, the one 

 being reversed for more ready comparison. Figure 7 is through 

 the anterior part of the coronoid process, and probably through 

 the most developed part of the surangular. On the inner side of 

 the dentary are three bones which I believe to be the svirangular, 

 articular, and angvilar respectively. Figure 8 represents a section 

 through the jaw a little behind the angle of the dentary. The 

 small bones on the inside of the dentary here are the angulai- and 

 the two posterior processes of the articular. 



From the five sections of the Theriodont jaw given from the 

 difierent specimens, a fairly good idea of the relations of the small 

 bones can be obtained. If they are arranged in the following 

 order, 7, 9, 10, 8, 6, we have a series of five sections from the front 

 of the coronoid process to near the posterior part of the dentary. 



