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THE AMERICAN NATURALIST. [Vol. XXXVII. 



Another fragment shows the upper portion of the maxillary 

 and the frontal united by their inner surfaces. The two portions 

 of the maxillary show that it was a thin, elevated plate, but rela- 

 tively not nearly so high as in Dimetrodon. The frontal differs 

 markedly from the frontal of Dimetrodon and all the specialized 

 Pelycosauria, it is nearly as broad as long, showing that the top 

 of the head was flat and broad. The orbital region is not pre- 

 served, but the anterior portion of the preorbital is, and this with 

 the posterior end of the maxillary serves to locate its position 

 with fair accuracy. The front edge of the preorbital is marked 

 by the edge of a pit or foramen which is similar to the pits 

 described by Seeley in Cynognathus and regarded by him as har- 

 boring a gland. In figure 3 is shown a partial restoration of the 

 skull of Diopeus illustrating the beginnings of the features found 

 in the more specialized Pelycosauria. 



ptf p 



The pterygoid bone is also interesting as showing the simple 

 beginnings of the peculiar pterygoid characteristic of the Pely- 

 cosauria. There is the same tripartite division of the bone, but 

 in the Dimetrodonts the posterior limb extends only as far back 

 as the quadrate and is united with it by cartilage while in Diopeus 

 it articulates strongly with the quadrate. The middle line is 

 destroyed but the external or ectopterygoid process is much 

 more slender and thin than the same process in the Dimetro- 

 donts. The outer surface which abuts against the lower jaw is 

 slightly rugose and the lower edge carries 15 teeth and alveoli. 



