OK THE MOLAR SERIES OF RHIKOCEROS TICHORHIKUS. 531 
6. M 3. The last molar, the most inconstant in form of all the 
teeth, in section sometimes trapezoid, sometimes triangular, is very 
frequently confounded with the other molars. In No. 856, of the 
Hunterian Catalogue of Fossil Mammals, it passes for * the ante¬ 
penultimate, or fifth molarin Nos. 859 and 865 for 1 the penultimate, 
or sixth molar of the upper jaw.’ 
The exterior surface is marked by three broad ill-defined costae, 
and is supported by two roots, the posterior of which is connate with 
the posterior of the inner surface. The posterior surface forms in¬ 
stead of a right a very obtuse angle with the exterior, from which it 
is divided by the third costa supported by the posterior and outer 
fang. The median groove is broad, deep and vertical. 
The anterior far exceeds the median collis in every dimension. 
The anterior valley, with entrance from 0*1 to 04 inches from base 
of crown, is deep, narrow and nearly parallel to the median line. 
Posteriorly the two ‘ combing plates ’ meet and insulate a portion of 
it, which Brandt considers the homologue of the posterior valley. 
But its external position, its opening into the anterior valley in the 
unworn tooth, and its being mapped off from it by the combing 
plates, show that it is the representative of the intercepted part of 
the anterior valley, that he terms Fossula externa. The median collis 
is very small and variable in form, sometimes being a rounded pillar, 
at others a plate with a trenchant free edge. In one specimen in 
my possession it is fully developed, and equal in size and inward ex¬ 
tension to the anterior collis. 
In the great majority of the specimens that I have examined, the 
posterior valley and collis are represented respectively by a faint 
groove on the posterior aspect and a small cusp * at its base. A 
series from WookeyHole, shows the gradual increase of both in size, 
until the groove deepens into an undoubted valley, walled in poste¬ 
riorly by a stout pillar 09 inches high, and in every respect the 
homologue of the posterior collis in the other molars. Deslongchampsf 
has arrived independently at the same conclusion with respect to the 
groove, which he terms, “ Vallon posterieur mais rudimentaire.” 
Measurements: 
(i) 
(2) 
(3) 
(4) 
Maximum 
1-85 . 
2-2 . 
1-6 
• 70 
Average . 
1*77 . 
1-99 . 
1-4 
. 644 
Minimum 
1*64 . 
1*88 . 
1-4 
. 64 
§ 6. Permakent Lower Molar Series.— -In the permanent 
lower molars the exterior aspect of the crown is divided by the median 
* In a specimen in the British Museum from Harrow dean this is absent, 
f ‘Memoire sur nombreux Ossements de Mammiferes Possiles trouvcs aux 
environs de Caen/ p. 57, pi. viii. fig. 4. 
