78 CONTRIBUTIONS FROM WALKER MUSEUM 
slit in the Dimetrodon gigas No. 1002, and is even doubtfully open in 
the Dimetrodon incisivus (?) No. too1. 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 
Fic. 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 
ee 
——— Ss 
