AKI) CLASSIFICATION OF THE FOSSIL KEPTILIA. 
225 
Subsequently,^' Sir R. Owen urged that the Theriodont dentition was monophy- 
odont, and he proposed to include in the order the European genera Britliopus, 
O)‘thopus, Rhopalodon, Deutcrosaurus, &c., which, so far as the dental characters are 
known, have very strong incisor teeth. But no evidence has yet been adduced, eitliei- 
in the skull or in the skeleton, that Theriodonts differ from Anomodonts as an ordinal 
group. For the value of the teeth as an ordinal character is small when so little 
specialized ; and the skull shows few differences in plan. 
The Structure of the Skull. (Plate 9, fig. 1.) 
The only region of the skull which is at present undescribed is the brain-case. 
There are four specimens in the British Museum which contiibute evidence as to its 
form and structure. There is also some reason to believe that in certain Dicynodonts 
the brain-case was very imperfectly ossified. A small skull divided vertically, which is 
partly figured by Sir R. Owen (‘ South African Catalogue,’ Plate XXVIII., fig. 4), gives 
no indication of a defined cerebral cavity. A similar skull, selected for its symmetry, 
and divided in the same way at my request, is equally free from evidence of a roof to 
the brain-case, though part of its floor is preserved. I am indebted to Dr. Henry 
Woodward, F.R.S., for having these joreparations made, and to Mr. Hall, the 
mason, for the skill with which the section was kept to the median line. The oniy bones 
shown in section (Plate 9, fig. 1) are the anchylosed basi-occipital and basi-sjohenoid 
at the back of the head, and the pre-maxillary and dentary in front. The basi-occipital 
{h.o.) is much less deej) in section than the basi-sphenoid (h.a.), which is perforated by a 
somewhat large carotid canal, extending downward and backward. Anterior to this, 
the posterior part of the bone sends a short process downward ; and the middle part 
sends a short wide process upward. From its anterior corner a short curve of faint 
narrow marking, which is not bone, extends forward and upward. A faint oblong 
mai'king below this occupies most of the interval between the basi-sphenoid and pre¬ 
maxillary, and might correspond to the vomer. From the upper border of the foramen 
magnum a line extends inward parallel to the basi-occipital. It is succeeded by a large 
oval mass defined in the same way by a sharp line. Upon this, and partly in front of 
it, is another oval mass, from which a thickish band is prolonged towards the nasal 
aperture. These outlines are in the position which the brain should occupy; but 
other evidence of the form of the brain does not lead me to suppose that the brain 
substance has been preserved. Thei’e is no reason for believing that the brain had 
this form or extended so far forward 4n a Dicynodont, though it certainly extended 
obliquely upward in the same way as do these markings. In the other half of the 
specimen the united basi-occipital and basi-sphenoid are cut slightly on one side of the 
median line, and here the superior surface is nearly straight and the basi-sphenoid is 
rounded in front (Plate 9, fig. 2). There is no trace of the pterygoid, which in all other 
* “ On the Order Tlieriodontia,” ‘ Geol. Soc. Quart. Jonrn.,’ vol. 37, p. 261. 
MDCCCLXXXIX.—B. 2 G 
