OSTEOLOGY OF THE SKULL OF THE DIMETRODON 79 
side of jugal, which at its lower end becomes sessile, and anteriorly 
with the maxillary. It covers the anterior and the upper portion of 
the outer process of the pterygoid. 
The bones of the facial region are very similar in position to those 
of previously described specimens, but there is shown a separate bone 
at the anterior end of the 
nasal, forming the pos- 
terior wall and a portion 
of the floor of the exter- 
nal nares. This occupies 
the same position as the 
bone called the septo- 
maxillary in Sphenodon 
by Howes and Swinner- 
ton. The bone has a 
very peculiar form, being 
bent at right angles so 
that the anterior portion 
forms the posterior half 
of the floor of the nares, 
and the posterior half 
forms the posterior wall. The two bones of the opposite sides ‘meet 
in the median line, so that they would close the nares; but the inner 
part of the posterior half is only one-half as high as the outer, so 
that the inner opening of the nares is elevated. The air entering 
the nares could not pass directly backward or downward, but first 
rose over the half partition, and then down into the mouth. The 
lower edge of the septo-maxillary joins the maxillary and premaxil- 
lary. The suture between maxillary and septo-maxillary is marked 
by two foramina. 
The section of the skull shows several peculiar conditions. ‘There 
are paired prevomers which are anteriorly united with the premaxil- 
laries. Passing backward, they are convex upward, so that the 
anterior portion of the mouth is vaulted. Opposite the maxillary- 
Fic. 4.—Posterior view of same skull. Letters 
as in Fig. 2. 
1G. B. Howes anv H. H. Swinnerton, “On the Development of the Skeleton 
of the Tuatera Sphenodon (Hatteria) punctatus,” Transactions of the Zodlogical 
Society of London, Vol. XVI (1903), Part I, No. 1, pp. 1-87, Plates I-VI, Figs. 18. 
