150 SIR RICHARD OWEN ON THE SKULL AND DENTITION 
discovered by our late Fellow-member, Mr. Charles Moore, in a 
bone-bed at Holwell, Somersetshire, are referred to, and are now 
accepted, as of a like geological age, the “* Rhetic.” 
I reproduce three of the figures, somewhat enlarged (Plate VI. 
8, 9, 10), of these teeth. They are molars, and the two fore-and-aft 
ridges of their grinding-surface are divided into a number of small 
tubercles * ; these, like the ridges, run in the same antero-posterior 
direction as in the tuberculate ridges of the upper molars of Jri- 
tylodon. 
Not any teeth of Tertiary or existing Mammals, known to me, 
show the multituberculate character of fore-and-aft ridges or raised 
portions of the grinding-suriace: the nearest approach is presented 
by members of the lower lissencephalous and lyencephalous groups 
(Chiroptera, Insectivora, Rodentia, Marsupialia); but the tubercles 
are fewer in number, relatively larger, and the lengthwise arrange- 
ment of both knobs and ridges is wanting, 
Another extinct mammalian genus, with molars of the same mas- 
ticatory proportions as in Tritylodon, has been discovered in the 
Oolitic slate of Oxfordshire ; it was described under the generic 
name Stereognathus by Mr. CharlesworthtT. The teeth were included 
in a portion of jaw, referred to a mandibular ramus, and two of 
them showed the characters of the grinding-surface. This unique 
fossil was kindly submitted to me, and the best-preserved molar I 
described as “ of a quadrate form, of very little height, and support 
ing six subequal cusps in three pairs, each pair being more closely 
connected in the antero-posterior direction of the tooth than trans- 
versely’. The dental generic distinction is shown in the minor 
subdivision of the summits of the ridges: but their number and 
course are the same ; “the crown of these molars might be described 
as supporting three parallel antero-posterior ridges; but each ridge 
in Stereognathus developes but two tubercles *’§. 
Now, this is the character more definitely expressed by the antero- 
posterior series of cusps in the quadrate upper molars of Tritylodon: 
its lower molars may show a nearer resemblance to those of 
Stereognathus. 
The dental characters of Tritylodon are thus repeated in extinct 
genera of Secondary periods, but are repeated, apart, in distinct 
genera. 
Microlestes shows the antero-posterior ridges of its multitubereu- 
late molars (Plate VI. figs. 8-10); but such ridges are limited to two 
in number. 
* «The inner side of the tooth is indicated by the more prominent part of 
the wall, which is divided into three tubercles; the outer side by the lower wall 
with indications of subdivision into smaller and more numerous tubercles.” 
Owen, ‘Researches on the Fossil Remains of Australian Mammals, with a 
Prefatory Notice of British Mesozoic Marsupials.’ 4to, vol. i. p. 6, platei 
figs. 1, 6, 7, 9, 19. 
t Quart. Journ. Geol. Soc. vol. xiii. (1857), p. 1, pl. i. 
t Op. cit. vol. i. p. 19. 
§ Op. cit. plate 2, figs. 27-30. 
