A DESCRIPTION OF THE GENUS DIMETRODON^ COPE. 45 



edge is folded upon itself, forming a long and deep pit on the outer side. The edge is 

 continued backwards horizontally, or even rising a little for nearly half the extent of the 

 plate, and then falls off rapidly to join the inferior edge in a sharp point. The middle 

 of this posterior edge is marked by a notch. It is possible that an anterior prolongation 

 of the quadrate overlapped this part of the pterygoid, as it does in Sphenodon. 



The massive part stands out from the plate-like portions ; supported by a rather 

 stout neck, it expands distally and its external face extended below the line of the teeth 

 and formed an opposing process to the coronoid of the lower jaw. The upper end of the 

 distal portion is injured on both sides, but probably bore a face for the ectopterygoid, 

 which must have been present, as there is a distinct face for such a bone on the inner face 

 of the jugal where it meets the maxillary. The ventral edge of the ectopterygoid process 

 is rounded and bears eleven comparatively large teeth, which are placed in distinct 

 alveoles and replaced from behind. 



The anterior part is horizontal, the inner edge excepted, which is turned vertically 

 upwards. The inner edges were close together and were united in the anterior part. 

 The lower portion is covered nearly completely with small conical teeth — those on the 

 outer and posterior region are the largest. A groove extends between this tooth-bearing 

 region and a ridge marking the origin of the vertical plate. There are a few small teeth 

 in the groove, and the ridge also bears a series of small teeth. 



The pterygoids articulate with the basisphenoid processes just posterior to a point 

 opposite the union of the ectopterygoid processes with the plate-like part. There are no 

 faces on the pterygoid corresponding to those on the basisphenoid processes. The union 

 must have been by ligament. The connection between the posterior processes with the 

 quadrate was also ligamentous. 



The palatines are strong bones, becoming more slender posteriorly. They are con- 

 nected with the maxillary by their entire external edge by suture. The articular face is 

 broadened and vertical. The inner portion of the ventral side of the palatines is covered with 

 small conical teeth. The region bordering the posterior nares is preserved and deter- 

 mines their position, their posterior ends being just behind the large canine tooth. 



The vomers are not preserved, but there are two small faces at the middle portions of 

 the posterior line of the premaxillaries showing where they were attached. They were 

 probably long and slender, paired and covered with small tubercular teeth. That the 

 vomers were directly connected behind with the anterior processes of the pterygoids, 

 excluding the palatines from the middle line, is very probable. 



The lower jaw is represented by three bones — the articular, coossified with the 

 angular, the dentary and a third bone, probably the supra-angular. The dentary contains 

 twenty-seven teeth, which are located on a thickened ridge in alveoles. The first tooth is 



