(2 
ie OSsTROLOGY OF THE SKULL OF THE PELYCO- 
SAURIAN GENUS, DIMETRODON. 
Durinc 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 too1, 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 root. 
As shown in Fig. 1, 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 S phenodon. 
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. 
 E. D. Cops, “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 ot 
the Mammals,” Anatomische Anzeiger, Vol. XIII (1897), pp. 109-20; tdem., “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. 
2 Op. cit. 
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