| OCR Text |
Show 1904.] OP THE THERIODONT MANDIBLE. 493 to be somewhat displaced inwards and forwards. On the right side it is displaced very 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 squamosal. 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 articulations of both jaws fairly satisfactorily (3). On each side the articular and angular 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. X X X V . , 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 squamosal. 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 surangular, articular, and angular 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 angular and the two posterior processes of the articular. From the five sections of the Theriodont jaw given from the different 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. |
