THE AMERICAN NATURALIST. [Vol. XXXVII. 



skull it differs from the specialized Pelycosauria. As the pos- 

 terior portion of the skull is destroyed it is impossible to say 

 with absolute certainty what was the condition of the temporal 

 arches, but from the other relations of the skull there seems no 

 reasonable doubt that they were as in Proterosaurus, i.e., with 

 two distinct arches. 



Cope describes the neural spines as elevated in all the species of 

 Theropleura, but this is not to be taken in same sense as the ele- 

 vation of the spines in the specialized members of the Pelyco- 

 sauria. The length of the elevated spines does not exceed two 

 or three times the length of the centrum where in Dimetrodon 

 it is as much as 28 times the length of its centrum. 



Diopeus leptocephalus. — The skull of this form is represented 

 by disarticulated fragments which are fortunately quite free from 

 crushing, and from the matrix. The fragment showing the tem- 

 poral region is made up of the quadrate and quadrat o-jugal, the 

 united squamosal and prosquamosal ; cemented to these are the 

 pterygoid and the epipterygoid almost in the natural position. 

 These fragments show that the temporal region, as figured by 

 Cope (Cope' 92), was very similar to the same region in Sphen- 



covered by the vertical extension of the quadrato-jugal. The 

 quadrato-jugal is much stronger than in Sphenodon and has 

 the form of a right angle with one arm vertical and the other 



