MORPHOLOGICAL REVISION OF THE SUBORDER. 



155 



the posterior edge is continued as a long ridge on the side of the centrum to the 

 posterior edge. The anterior face is elongate vertically, but it has been crushed so 



that the form is ob- 

 scured. The spine is 

 low, but it was ver>' 

 heavy, curviug forward 

 over the anterior face; 

 the posterior zygapo- 

 pliyses are large, but 

 the anterior ones are 

 small. The posterior edge of the spine is 

 very wide, extending out over the sides of 

 the centrum. For measurements see the 

 original description, page 69. 



Position and relationships : It is most 

 unfortunate that the vertebrae and limb of 

 this form are not known. The evidence of 

 the skull shows that it has little in com- 

 mon with the rest of the Pelycosauria. The 

 probable similarity of habits to Placodus has 

 been pointed out above, and a comparison 

 of the skulls shows many points of resem- 



Fig. 72.— Diagram of ihe palalal surface of Placodus. blanCC. Thcse are bcSt shown in a COm- 

 After V. Huene. „ .• . vi 



c- 72 r. I L I £ L 1 11 c parative table. 



rig li. — Diagram ot tiie posterior surface of ttie skull or *■ 

 Placodus. After v. Huene. 



Lettering as in figs. 66 and 67. 



Placodus. 



Skull low and broad. 



A single temporal vacuity. 



Incisor teetfi chisel-shaped; projecting forward from 



the skull. 

 Maxillary teeth flat and adapted to crushing or 



grinding. 

 Posterior nares a single opening formed by the pre- 



vomers and premaxillaries. 

 Palatine teeth few and large. 

 Teeth of the lower jaw reduced to the incisors and 



crushing teeth. 

 Pterygoid without external process. 

 Interparietal and epiotic present. 

 Occurring in the Triassic. 



Edaphosaurus. 



Skull low and broad. 



Probably two temporal vacuities. 



The same. 



Maxillary teeth thin and sectorial. 



Posterior nares a single opening formed by tfle pre- 



vomers. 

 Palatine teeth numerous and smaller. 

 Lower jaw with teeth on the border as well as the 



crushing teeth. 

 Pterygoid without external process. 

 Interparietal and epiotic present. 

 Occurring in the Permian. 



There is not sufficient evidence to warrant any conclnsion that there 

 is an actual relationship between EdapJiosatinis and Placodus^ but the simi- 

 larity of the two is certainly very suggestive. If Placodus is a true Synap- 

 sidan form, related to the Anomodonts, as held by many authors, it could 

 have no connection, genetically, with Edaphosaiinis., but it must have passed 

 in any case through a similar stage of evolution in the growth of the pala- 

 tine and dentary crushing teeth and the loss of the teeth on the edges of the 



