OSTEOLOGY OF THE SKULL OF THE DIMETRODON 



6"^^ 



The determination was made on a partially preserved skull of 

 Dimetrodon incisivus, and a series of unfortunate conclusions have 

 been drawn from this erroneous determination. The present speci- 

 mens show that Cope was correct in his determination of the quadrate 

 as an elevated bone, and also demonstrates its remarkable similarity 

 in position and relations to the quadrate of Sphenodon. 



Figs. I, 2, and 4 show the general form and relation of the quad- 

 rate. It is an elevated, thin plate of bone ending freely above, articu- 



FiG. I. — Left side of the skull of Dimetrodon gigas. About one-fourth natural 

 size. Full length of skull, 46 cm; 



lating with the pterygoid anteriorly, and the quadrato-jugal, squamosal 

 and paroccipital posteriorly. The lower end is terminated by two 

 elongate articular condyles, which run almost parallel antero-posteri- 

 orly, but are slightly convergent anteriorly. The inner condyle 

 stands out from the side of the bone, and its inner side articulates 

 with the posterior end of the pterygoid. The outer condyle projects 

 beyond the posterior edge of the bone, and its upper surface is flat, 

 forming a sort of shelf, to the upper side of which is articulated the 

 lower end of the quadrato-jugal. 



The quadrato-jugal is a very slender plate of bone that articulates 

 with the posterior edge of the quadrate for its full length. Above, the 

 quadrato-jugal passes between the squamosal and prosquamosal, and 

 articulates with the parietal; below, it is separated from the quadrate 

 by a fair-sized quadrate foramen. 



