OSTEOLOGY OF THE SKULL OF THE DIMETRODON 307 



slit in the Dimetrodon gigas No. 1002, and is even doubtfully open in 

 the Dimetrodon incisivus (?) No. looi. The edges of the bones 

 adjacent to the opening are thinned, and in case where the opening is 

 uncertain there is clear evidence of the thinness of the roof of the 

 skull. If this superior temporal opening is just appearing, as seems 

 certain, we have confirmatory proof of the origin of the temporal 

 arches by a process of natural trephining of the completely roofed 

 skull, as proposed by Baur. It is important to notice that the bones 



Fig. 3. — Palatal view of the same skull. Letters as in Fig. 2. 



have arranged themselves in the position of the perfect arches before 

 the openings appear. 



On the posterior face of the skull the remnants of fairly strong 

 stapes was found in position. Unfortunately, neither end was pre- 

 served, so that it is impossible to confirm Cope's description of the 

 anterior end of the Pelycosaurian stapes. 



On the inferior face of the skull the position of the pterygoids and 

 other bones is confirmed, but the external processes of the pterygoids 

 are shown to have been located farther forward than supposed — at 

 the posterior end of the maxillaries. It is determined that there were 

 no posterior palatine openings between the palatine and maxillary. 

 Anteriorly the nares are separated by the paired prevomers; the sides 

 of the prevomers are marked by rugosities at the inferior opening of 

 the nasal canal. 



The ectopterygoid (transverse) is made out for the first time. It 

 is a short bone, articulating with a strong, curved ridge on the inner 



