OF THE PEr.YCOSAURIAN GENUS DIMETRODON. 7 



posterior portion and that the orbit was much nearer to the middle of the skull. The 

 elevated facial region wliile it is one of the most characteristic features of the skull 

 was not carried to the extent figured by Cope in his restoration of the closely related 

 genus Naosaurus ('92). 



Below is a detailed description of the skull in which it will be seen that in most 

 particulars it bears a striking likeness to the skull of Splicnodoii so that in most parts 

 the two can be compared directly. 



The quadrate, PI. V, fig. 1 : This is a thin plate of bone of considerable vertical 

 extent reaching nearly half the height of the posterior portion of the skull, but not 

 reaching such a great antero-posterior length as the same bone in Sphenodon. The 

 articular portion consists of two condyles elongate in the antero-posterior direction and 

 with their main axes converging slightly as they advance so that all motion of the jaws 

 was rigidly limited to the vertical plane. The outer condyle is the more slender and 

 lies almost in the plane of the upper portion of the bone ; posteriorly it extends beyond 

 the main part of the bone as a prominent process with its upper face flattened into a 

 sort of shelf to which is attached the lower end of the quadrato-jugal. The inner con- 

 dyle is stouter and is offset from the body of the bone. The posterior edge of the quad- 

 rate is rounded and gives attachment through its length to the quadrato-jugal, but just 

 above where the quadrato-jugal joins the upper surface of the inner condyle the two 

 are separated by a good sized foramen, the foramen quadrutum. This foramen serves 

 as an important landmark in the skull ; it is not present in the Cotylosauria ; it is 

 probably present in the primitive ^I'c/iostmria {= Diaptosauria, Osborn) although it 

 has been demonstrated only in the Pelycosauria and Rh;/ncocephalia vera; it is present 

 in the Theropodous Dinosaurs, the Icthjosaurs and the Phijtosaurs ; it is absent in the 

 Crocodilia, the Pterosaurs and the S(piamata. 



The posterior end of the pterygoid overlaps the quadrate on the inner side, the 

 lower edge extends back almost to the posterior limit of the bone and is attached to 

 the inner side of the inner condyle. 



The quadrato-jugal: The quadrato-jugal occupies a relatively unimportant posi- 

 tion ill the skull. It is a very thin plate of bone, with its lower end and posterior edge 

 attached to the quadrate as described above. The upper end becomes very sharp and 

 is wedged in between the prosquamosal and squamosal and comes in contact with the 

 parietal. It is separated from any contact with the jugal by the descending process 

 of the prosquamosal, as described below, and in turn it separates the prosquamosal 

 from the squamosal, thus occupying a unique position among the reptiles. The 

 position of the quadrato-jugal is not anomalous, however, for if the upper end were 

 withdrawn from contact with the parietal by shortening, the prosquamosal and 



