FEB 6 



TRANSACTIONS 



AMERICAN PHILOSOPHICAL SOCIETY 



ARTICLE I. 

 THE MORPHOLOGY OF THE SKULL OF THE PELYCOSAURIAN GEXUS DIMETRODON. 



[PJates 1-7.] 



BY E. C. Case. 



(Read October 7, 1904.) 



The following description is based on four skulls in the collection of the University 



of Chicago, bearing the numbers 1, 114, 1001 and 1002, in the collection of vertebrate 



fossils of that University. All four of the skulls were discovered and collected by the 



author of this paper, the first two in the summer of 1896 and the last two in the 



summer of 1903. All are from practically the same horizon, the Permian beds of 



Texas, in Archer and Baylor Counties. Numbers 1 and 1 14 have already been pretty 



fully described by the author (Baur and Case, '99, '03), and only such portions are 



here redescribed as are necessary to supplement the material afibrded by specimens 



1001 and 1002. The last two consist of singularly perfect skulls, showing the complete 



anatomy of the temporal arches, a region which, by reason of its fragility, is almost 



always destroyed in the process of fossilization. The two skulls were accompanied by 



considerable i^ortions of the skeleton in both cases, but w'ere preserved in a very 



different manner. Number 1001 was discovered in a soft, friable shale, carrying much 



gypsum and many impre.ssions of ferns, with a considerable quantity of lignite. The 



nature of the matrix caused the bones to be badly broken and in some parts rotted by 



the gypsum, but all were preserved in place, and the skull and lower jaws were 



continuous with the skeleton. The processes of collection and preparation have been 



very tedious, but when once the bones were joined fhey could be cleaned from the 

 A. p. S.-XXI. A 



