Page 468 - My FlipBook

478 JJESTAL ANATOMY.

tern is substantially the same as in Plienacodus, with the slight excep-
tion that the cusps are more elevated and laterally flattened, and the
external rib is very small or absent in Hyrucotherium. In the nearly
allied genus PHolopIius, which I suspect to be the same as Orohippus of
Marsh, the last or fourth premolar below is like the true molars in form,
and is (piadritubercular, while the genus Lopldotlierium has the third
and fourth premcjlars below, like the true molars.
In the second section of this family the external lobes of the superior
molars are laterally flattened and intimately blended together, so as not
to be well distinguished. Of these the anterior .is much the smaller, and
is convex externally, whereas the posterior is large and concave without.
The intermediate tubercles no longer exist as such, but form jjrominent
crests which connect the external with the internal cusps, crossing the
crown somewhat obliquely. In the lower molars the external and inter-
nal cusps are also connected by crests, giving the typical lophodont pat-
tern. As a rule, the premolars are trilobed, and the molar formula is
Pm. ^, M. , but in one genus (Dilophodou), recently described by Prof.
f
Scott, there are only three premolars in the lower jaw. In another
genus, lately described by the same author under the name of JJcsina-
totherium, the third and fourth upper premolars are like the molars,
and are four-lobed.
The tapirs form another nearly related family (Tapiridce), which no
doubt sprang fr and canines are like those of the Lophiodoiitklce, but the canines in the
lower jaw of the living forms are somewhat procumbent. Tlie third and
fourth premolars in the upper jaw are like the true molars, which dis-
play the four cusps connected by cross-ridges remarkable for their trans-
verse direction in contrast with the oblique crests of some of the preced-
ing family. The two external lobes are likewise different in their
subequal pro})ortions, both being convex externally and well separated
from each other.
The lower premolars except the first are like the molars. The exter-
nal and internal lobes are connected by strong cross-crests, which are
as much elevated as the cusps themselves, and there is no ridge crossing
from the postero-external to the antero-internal lobe, as in Ht/racothe-
rium and Phcndcodns.
From this family we pass to the rhinoceros section of the sub-order.
In accordance with what the philosopliic student of the living forms
would be led to anticipate, this section pertains to a later geologic period
than the preceding, and not unnaturally would he seek for the connect-
ing linkb' between them and that section of the Lopthiodontidce, in which
the external lobes are flattened. Thi-ough the researches of American
palaeontologists we are now in a position to fully comprehend all the
more important steps in the evolution of this group, and I fail to recall
in the whole range of vertebrate palaeontology an instance in which the
demands of the evolution hypothesis are more completely satisfied than
in the present one.
The molar formula of the rhinoceros is Pm. ^, M. the usual num-
f,
ber in jK-rissodactylcs ; but, as regards the incisors and canines, the great-
est variability is to be observed. In the two-horned African species