108 PROCEEDINGS OF THE GEOLOGICAL society. [Noy. 3, 
angles of the two outer lobes do not coalesce upon a large median 
protuberance, but are quite separated by a minute intermediate basal 
tubercle ; the basal ridge is continued round the mner side of the 
crown, and the outer sides of the outer lobes are much more protu- 
berant at their middle part in Merycopotamus than in Hyopotamus. 
The teeth compared in the last two genera are of the same size; that 
of the Anthracothertum magnum is nearly twice as big; and that of 
the Cheropotamus is one-third less than the corresponding molar of 
Hyopotamus bovinus. 
The upper molars of the Dichodon, especially the second, resemble, 
in the general form of the crown and the particular shape of the four 
lobes, the tooth of the Hyopotamus: the outer sides of the lobes m 
particular have nearly the same slight degree of general concavity with 
the median longitudinal rising: the opposite angles of the two outer 
lobes equally meet upon an external median basal prominence, but at 
an acute angle, and the prominence itself is not so convex or bulging, 
and it also developes a small cusp on each side the entering angle. 
Moreover, the (antero-intermediate) lobe (2) is equally wanting in Dz- 
chodon as in Merycopotamus, and the enamel, which is smooth and 
polished in Dichodon, is strongly wrinkled in Hyopotamus. ‘The ge- 
neric distinction between Hyopotamus and Dichodon, which I should 
have regarded as quite sufficiently marked by the above-described 
differences in their upper molar teeth, is most satisfactorily and re- 
markably manifested in the dentition of the lower jaw. 
The upper molars of the little Hyracotherium* resemble those of 
the Cheropotamus more than they do those of the Hyopotamus ; 
but they appertain to the same general type of configuration; and 
this type—of quadrate crown, with four principal pyramdal and 
more or less distinctly trihedral lobes, divided by deep valleys, not 
filled up by cement, but, in some genera, interrupted with minor 
tubercles and ridges,—characterises a great natural group of Ungu- 
lata, most of the members of which are extinct, but which tend to 
fill up, in the zoological series, the wide interval that now divides the 
Peccari or the Hippopotamus from the Ruminants. 
The generic or subgeneric modifications of structure at present 
recognized in this great natural group are signified by names given 
to the partially restored genera :— Anthracotherium, Hyopotamus, 
Merycopotamus, Hippohyus, Cheropotamus, Adapes, Dichodon, 
Hyracotherium, Dichobunes, and Anoplotherium, to which also we 
ought perhaps to add Calicotherium. Hippohyus and Cheropota- 
mus seem to have stood nearest to the existing Peccari and the Hog- 
tribe ; Anthracotherium or Merycopotamus were, perhaps, more 
nearly allied to Hippopotamus. Cuvier thought that the Anoplo- 
therium bore a close affinity to the Camelide, and Dichobunes seems 
to have approached the Musk-deer (Moschide). 
antero-internal lobe not to have been developed, and its place in the ruminant 
molar to be now occupied by the greatly developed accessory lobe p; or by sup- 
posing this lobe not to have been developed, aud the antero-internal lobe to be 
the homologue of the lobe z in the ruminant upper molar. 
* British Fossil Mammals, p. 422, fig. 166. 
