THE OSTEOLOGY OF THE SKULL OF THE PELYCO- 

 SAURIAN GENUS, DIMETRODON. 



During the summer of 1904 the author collected in the Permian 

 beds of Texas two skulls of the genus Dimetrodon belonging in the 

 suborder Pelycosauria. These skulls were in an excellent state of 

 perfection, which permits the completion of previous descriptions and 

 the correction of some errors. 



Especially valuable is the fact of the preservation of the temporal 

 arches, permitting a description of this region, which has been 

 hitherto only partially known and falsely interpreted. The two 

 skulls are numbered looi, Dimetrodon incisivus ( ?), and 1002, 

 Dimetrodon gigas, of the University of Chicago collection of fossi 

 vertebrates. The specimen of Dimetrodon gigas was almost perfectly 

 •preserved, only a portion of the temporal arches of the left side and 

 the middle portion of the epipterygoid being lost. The larger part of 

 the following description is taken from it; some details, and the 

 description of the lower jaws, are added from specimen looi. 



As shown in Fig. i, the skull has proportions much like those of 

 the modern lizards or the carnivorous Dinosaurs. The eyes are not 

 located so far back in the head, and the facial region, while elevated, 

 does not bear the great disproportion to the skull shown in previous 

 restorations.^ 



The quadrate of Naosaurus was correctly interpreted by Cope as 

 an elevated element similar to the same bone in the modern Sphenodon. 

 This was later denied by Baur and Case,^ and the statement was made 

 that the quadrate was a depressed bone completely surrounded by 

 the bones of the temporal region, and in this regard similar to the 

 African Theriodonts. 



I E. D. Cope, " On the Homologies of the Posterior Cranial Arches of the Reptilia," 

 Transactions of the American Philosophical Society, Vol. XVII (1892); G. Baur and 

 E. C. Case, "On the Morphology of the Skull of the Pelycosauria and the Origin of 

 the Mammals," Anatomische Anzeiger, Vol. XIII (1897), pp. 109-20; idem., "The 

 History of the Pelycosauria, with a Description of the Genus Dimetrodon," Transac- 

 tions of the American Philosophical Society (2), Vol. XX (1899), pp. 1-58. 



^ Op. cit. 



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