1847.| OWEN ON EXTINCT ANTHRACOTHERIOID QUADRUPEDS. 113 
cotherium, the outer sides of the inner lobes are convex ; and in dn- 
thracotherium the inner sides of the outer lobes are ridged. The 
second true (penultimate) lower molar (m 2, figs. 1-3) consists in 
Hyopotamus, as in Anthracotherium, Cheropotamus and Dichodon, 
of two primary divisions, each subdivided into two pyramidal lobes : 
these are also higher, narrower antero-posteriorly, and sharper than 
in Anthracotherium ; and the oblique ridge forming the anterior angle 
of the postero-external lobe o', instead of crossing the transverse val- 
ley to terminate upon the antero-internal lobe 7, as in Anthracothe- 
rium, terminates at the internal end of the transverse valley: thus 
each pair of lobes are more distinct from one another than in Anthra- 
cotherium, and both in this respect as well as in the general form of 
the lobes, Hyopotamus more resembles Dichodon than it does An- 
thracotherium. The tooth in question differs from its homologue in 
Dichodon by the absence of the accessory cusps at each side of the 
inner base of the mner lobes, and in the irregular shape, instead of 
the smooth convexity, of the outer sides of the same lobes. The 
penultimate molar further differs from both that of Dichodon and 
that of Anthracotherium in having merely a narrow basal ridge at 
the back part of the crown, which does not develope a tubercle there. 
Such tubercle is distinctly shown in both fig. 2 (Anthr. magnum) and 
fig. 5 (Anthr. minimum) of pl. 80. of the ‘Ossemens Fossiles’ (tom. 
cit.), as well as in M. de Blaimville’s figure of the fine lower jaw of 
the Anthracotherium, which he supposes to be Anthr. magnum, from 
Auvergne. There is no basal ridge along either the inner or outer 
sides of the crown of the penultimate molar of Hyopotamus: the 
anterior basal ridge rising obliquely from the external to the internal 
side, developes a tubercle at the antero-internal angle of the base of 
the crown. The outer sides of the two outer lobes are more convex, 
the inner sides of the inner lobes less convex in Hyopotamus than in 
Anthracotherium. There is a small tubercle at both the outer and 
the inner end of the transverse valley, which is much deeper at the 
outer than at the inner side of the crown: the imner tubercle attaches 
itself to the anterior rather than to the posterior of the two inner 
lobes ; the ridge at the inner side of the transverse valley of the crown 
of the same tooth in Anthracotherium is continued upon the posterior 
of the two inner lobes. When it is remembered what important 
anatomical differences are associated with the mere addition of an 
accessory tubercle to one of the molar teeth in the genus Semnopi- 
thecus, as compared with Cercopithecus, in the Quadrumanous order, 
the necessity of the above-detailed comparisons will be appreciated. 
In all the features by which the penultimate and last molars of the 
Anthracotherium differ from those of Hyopotamus, they approach 
the characters of the same teeth of Cheropotamus, from which there- 
fore Hyopotamus in a like degree deviates. The first true (ante- 
penultimate) molar of the Hyopotamus (Pl. VIII. m 1, figs. 1-3) is 
smaller than the second, but closely resembles it in structure: the 
lobes are lower, being more worn, but the differential characters from 
Anthracotherium on the one hand and from Dichodon on the other 
are as well-marked as in the case of the second molar tooth, 
