OF TRITYLODON LONGAVUS. 151 
Stereognathus repeats the three parallel antero-posterior ridges ; 
but the tubercles on each ridge are limited to two in number. 
In Tritylodon, these characters are combined. 
Such testimony was, I confess, a relief to me; for my experience 
of the air-breathing Vertebrates of the old Secondary formations of 
South Africa had been, throughout its great extent, so uniform 
in respect of their limitation to the cold-blooded series, that I hesi- 
tated, at first view of the subject of the present paper, to acknow- 
ledge it as, in the scale of life, so far above the series of fossils with 
which it was associated. The more obvious characters of cranial 
structure and of dentition, even in the comparatively undeveloped state 
of the fossil skull, could only be interpreted, however, as in the 
foregoing details. 
The mammalian characters of Tritylodon may be briefly summed 
as:—Dentition of scalpriform incisors divided by along interval from 
a series of broad-crowned tuberculate molars, implanted by distinct 
roots (fig. 3, m5), showing end of growth (the Leporide and some 
other mammalian genera have rootless grinders); no trace of 
provision for succession of teeth ; osteology of skull, as above de- 
scribed, showing mammalian conformity with the characters of the 
dentition. 
Having obtained the sanction of Dr. Exton, I took our experienced 
mason, Mr. Barlow, into council, instructed him as to the portions 
of the fossil which ‘might be safely subjected to his skill, and the 
result is the specimen which I have now the honour to submit, with 
an interpretation of its characters, to the Geological Society. 
DESCRIPTION OF PLATE VI. 
Upper view of the skull of Tritylodon : nat. size. 
Under view of the same: ditto. 
. Side view of the same: ditto. 
Front view of nasal end of the same: ditto. 
Outer side view of right fourth upper molar of Tritylodon: three 
times the nat. size. 
. Inner side view of the same: ditto. 
. Grinding-surface of the same: ditto. 
. Outer side view of an upper molar of Microlestes Mooret: ditto. 
. Grinding-surface of the same molar: ditto. 
. Grinding-surface of an upper molar of Microlestes rheticus: ditto. 
. Grinding-surface of a molar of Stereognathus ooliticus: ditto. 
(Figs. 9-11 have been drawn at right angles to fig. 7.) 
a 
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ht : 
FHODONSD TF CD 
DIscusston. 
The PresipENT remarked upon the singularity of the form of the 
skull and of its dentition, and on the great interest of the com- 
munication. 
Mr. LypExxer said that the interest of this specimen rested in 
the remarkably differentiated character of the dentition. The only 
groups it would seem to be connected with are the Marsupials and 
the Rodents. The high specialization is most remarkable, and it 
does not seem to throw any light on the common origin of the 
existing groups of Mammalia. 
