A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 57 



of palate was developed from the Rhynchocephalia, through the Belodonts and the 

 Teleosaurs. It is possible, that the Gomphodontia originated from the Proganosauria. 

 The question to be solved now is : What is the single temporal arch in the Gomphodontia 

 and Mammalia f There are two possibilities ; it represents either both the upper and 

 lower arches united, or the lower one alone, the upper one being reduced. 



" Seeley, 67 in his paper on the Cynodontia, gives a lateral view of the skull of Cynog- 

 nathus crateronotus. There is a large supratemporal fossa, but besides, there is a small 

 vacuity", between the squamosal and the jugal. If this vacuity is natural, it can only 

 represent the infratemporal fossa. By the disappearance of this infratemporal fossa a 

 single temporal bar would result. Further researches have to decide this very important 

 question." 



In a paper by the junior author 66 an attempt has been made to show that the vacuity 

 here mentioned in the temporal region of Cynognathus is not a fracture, but the final 

 stage in the final union of the two arches to form the zygoma of the mammals. 



" A specimen of Cynognathus crateronotus, figured by Seeley, shows an opening 

 between the upper and lower arches which was uncertain in origin, there being some 

 reason to suppose it to be the result of an injury to the specimen, but a study of the figure 

 of Procolophon, given by Seeley, shows the same condition. The enormous quadratojugal 

 (called squamosal by Leydekker) joins the jugal in front, which in turn joins a slender 

 element by its anterior superior corner ; this element runs backwards, forming the lower 

 and back portion of the orbit, and is undoubtedly the postorbital. Behind this element 

 is another bone, the squamosal, or squamosal + prosquamosal, which rests upon the 

 quadratojugal below ; between all these elements is a small cavity, exactly as in Cynog- 

 nathus. It is hardly probable that a break would occur in the same place in the two 

 specimens, and so they are considered as showing the final stages of the union of the two 

 arches to form the mammalian zygoma." 



If these conclusions be correct there is an uninterrupted chain of forms from the 

 most primitive of the Pelycosauria with two widely separated arches to the Gomphodontia 

 with a single arch made up of the union of the two and in all probability to the 

 Mammals also. With this progress in the development of the zygomatic arch goes a 

 series of changes in other regions of the skull as the gradual assumption of the tuberculate 

 forms of the teeth and the reduction of the quadrate bone. 



The Pelycosauria now assumes a most important position in the mammalian-reptilian 

 phylum. As stated in the paper last mentioned, 66 the group seems to be the beginning 

 of the long line of forms that culminated in the Mammals. 



Completed September, 1898, E. C. C. 



State Normal School, Milwaukee, Wis. 



A. P. S. — VOL. XX. H. 



