CLASSIFICATION OF THE THERIODONTIA . 79 



upwards, and a little forwards. The postfrontal is very 

 small, the preparietal is absent. The maxilla is very short and 

 deep, there is no step in the upper jaw. The prefrontal is short 

 and probably small. I have already shown that the basicranial 

 and otic regions are very advanced (Proc. Zool. 8oc. 1914, 

 p. 1028). The epipterygoiils are widened and flat, very much 

 as in Diadem odon. The posterior end of the quadrate ramus of 

 the pterygoid no longer reaches the quadrate. The pterygo- 

 parasphenoidal girder, instead of having a flat ventral surface 

 from which a deep median crest rises, is rounded, with the crest 

 represented by a low fillet. 



The palate is most conveniently compared with that of 

 Gorgonops, which represents an earlier stage leading to it. 



The great median groove is deepened and its roof is entirely 

 formed by a median vomer, which presumably represents a 

 forward growth of the posterior median bone of the Gorgonops 

 palate. Anteriorly the ridge rising from the vomer in Arcto- 

 gnathus suggests that there was a soft secondary palate into 

 which a secondary bony plate may have grown out in the 

 concealed part of the palate. 



The tooth-bearing roughened area of the anterior ramus of 

 the pterygoid lies much further back than in Gorgonops, and the 

 pterygoid no longer reaches the posterior nares. 



The quadrate of Arctognathus is much smaller than that of 

 Scymnognathus. 



The conversion of Arctognathus into a "Cynodont" like Gyno- 

 gnatlms demands only the following changes : — Still further thin- 

 ning of the basis cranii, further reduction of the quadrate wing of 

 the pterygoid, the development of a connection between the 

 quadrate wing of the epipterygoid and the paroccipifcal ; further 

 retraction of the anterior ramus of the pterygoid, so as to reduce 

 the roughened areas to a pair of small knobs on each side of the 

 posterior end of the median groove ; a little reduction of the 

 ectopterygoid ; the development of secondary plates from the 

 maxilke and palatines in the existing soft secondary palate; 

 the conversion of the narrow intertemporal area into a. sagittal 

 crest, to increase the length of the temporal muscles ; the loss 

 of the postfrontal, and a further reduction of the frontal and 

 prefrontal, leading to an increase in size of the posterior part of 

 the nasals. These changes are all in the same direction as those 

 which convert a Pelycosaur like Varanosaurus into a Theriodont 

 like Gorgonops, and an animal like Gorgonops into a form like 

 Arctognathus , and are, on the whole, smaller than those which are 

 necessary to carry out the earlier improvements; in fact, Arcto- 

 gnathus, which is technically a Gorgonopsid, is structurally closer 

 to Cynognathus than it is to Gorgonops. 



Amongst other advanced forms allied to the Gorgonopsids and 

 coming from the Gist#cephalus-z.one are Cynosuchus and Whaitsia, 

 which have both been excellently described, though not com- 

 pletely figured, by Haughton. 



