OF TRITYLODON LONG&VUS. 149 
the mid ridge, 6, three on the inner ridge, ¢, and two on the outer 
ridge, a. Of the tubercles on the mid ridge, 6, the hindmost is 
smaller than the others; and, in the fourth molar, a minute one 
projects between the bases of the first and second tubercle. There 
is a similarly minute tubercle behind the base of the second normal 
one of the outer coronal ridge. But the number of the tubercles 
which first catches the eye on the middle and internal ridges of 
the crowns of the best-preserved molars is “ three”: and this has 
suggested the generic name, which must not be interpreted to mean 
that there are but three tubercles on each tooth, but to indicate 
that the tri-tuberculate pattern prevails, more or less, on each of 
the three ridges of the singularly complex and unique grinding- 
surface of the molars of the present oldest known genus of Mammals. 
The sixth and last molar—tlast so far as the mutilated condition of 
the post-maxillary part of the skull permits a judgment—is rather 
smaller than the rest, and the subquadrate transverse shape of crown 
here inclines to a trihedral shape, with the apex outward. ‘The 
threefold division of the crown is, nevertheless, as strongly marked ; 
but one knob of normal size only is developed on the outer ridge, 
flanked by a minute tubercle on each side of its base. The normal 
tubercles of the mid and innerridges are two in number; the larger one 
on the inner ridge repeats or completes the transverse array of three. 
In an outer side view (Plate VI. figs. 3 & 5) these molars, from 
the second to the fifth inclusive, present a bilobed character; the 
anterior tubercle is reduced to the basal cusp in the sixth molar. 
The inner or mesial side of the 2-5 molars gives a trilobate outline, 
fig. 6. I subjoin views of the best-preserved, third, right-side 
molar, enlarged three diameters, showing its outer, 5, inner 6, and 
masticating, 7, surfaces, the characters of which, in my odontological 
experience, are unique. 
If reference be made to the 6th Volume, 2nd series, of the ‘ Trans- 
actions’ of our Society, p. 203, pl. xxi., the enlarged view, fig. 4, of 
the upper molars, grinding-surface, of Hyracotherium lepormum 
may suggest a resemblance to those of Tiritylodon. But the middle 
multicuspid ridge is represented by only two small tubercles, and 
both the outer and inner risings of the crown are also limited to two 
tubercles. Moreover it will be noticed that such degree of approach 
to the grinding-pattern in T’ritylodon is restricted to the three true 
molars characteristic of the placental Ungulates. The repetition of 
the multicuspidate character in five consecutive grinders recalls the 
lower marsupial numerical type of dentition (Thylacotherium, e.g.). 
Nevertheless m1 & m2 may be “ premolars.” 
In the further course of comparisons, I have found the nearest ap- 
proach to the masticating character of T’ritylodon in teeth of Mammals 
of equal, or nearly equal antiquity. These evidences are afforded by 
two extinct genera. The first, from the Keuper of Diederloch, consist 
of teeth or a tooth-crown, on which the discoverer, Plieninger, in 
1847, founded his genus Microlestes*. Teeth of like character 
* ‘Jahreshefte des Vereins fiir vaterlandische’Naturkunde in Wirttemberg,’ 
8vo, Bd. ii. p. 164, Taf. i. figs. 3 & 4. 
