A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 55 



South African forms : 



Gynodontia.* Cynochampsa. 



Cynognathus. Nythosaurux. 



Galesaurus. Scaloposaurus. 

 Tigrisuchus. 



There is little doubt that a considerable synonymy exists among the American 

 forms. A majority of the genera were described from characters of the vertebra? alone 

 and were founded on isolated vertebrae or on small series from separate regions of the 

 spinal column. Thus Lysorhophus was founded on the fact that the neural arch is sepa- 

 rate from the centrum, and that the capitular articulation of the rib is confined to the 

 preceding intercentrum. In Theropleura the neural arch was free from the centrum and 

 the capitulum of the rib was attached to the anterior end of the centrum. In Dimetro- 

 don the " capitulum extended downwards and forwards to the anterior end of the centrum, 

 but (as far as observed) there is no facet." In Embolophorous the capitulum of the rib 

 is definitely described as joining the preceding intercentrum. As has been shown in the 

 description of the skeleton of Dimetrodon, all of the conditions of the capitular articula- 

 tion described in these genera are found in different parts of the column of the single 

 specimen. The freedom or attachment of the neural arch within the group is hardly 

 more than a character of age. Metarmosaurus was founded on the shortness of the 

 centrum and the absence of the capitular face, exactly the conditions that are found in 

 the posterior lumbar and the anterior caudals of Dimetrodon. There is little doubt that 

 many of these genera are well founded, but only the consideration of a large amount of 

 material will make it possible to clear up the synonymy. 



The position of Theropleura is doubtful from the fact that the teeth are said to be 

 the largest in the middle of the molar series, a character that is not common in the 

 Pelycosauria, and is quite common in the American forms of the Pareiasauria. The 

 same thing is true of the Russian form Deuterosaurus. 



In our preliminary paper 28 the affinities of the Pelycosauria were discussed, as follows : 



" There cannot be any doubt that Dimetrodon is nearest to the Rhynchocephalia and 

 Proganosauria {Palazohatteriidoz) . The structure of the skull, the vertebral column, and 

 the humerus are of the same type. The presence of a distinct squamosal and prosqua- 

 mosal is of special interest. The same condition we find in Sapheosaurus H. v. Meyer 

 {Sauranodon Jourdan) of the Jurassic Sapheosauridos ; and there is very little doubt that 

 these two elements are also present in Palosohatteria Credner. The bone marked squa- 



* The group Gynodontia was considered by Seeley as worthy of separation from the remainder of the forms 

 by the development of lateral tubercles on the teeth. To this character may be added the union of the superior 

 and the inferior temporal arches, and the more or less complete obliteration of the superior temporal foramen. 

 The Gynodontia are so close to the other forms of the group that it is impossible to separate them off even as a sub- 

 order, but they certainly do demand recognition for the advance in the development of the teeth and of the condi- 

 tion of the cranial arches toward the type of the Gomphodontia. 



