Upper Jaw.--The incisors, three in number, are placed close together
in a nearly straight line, which does not make so great an angle with
the line of molars as in _Palæosyops_. They increase regularly in
size from the first to the third, which is very large. They all have
stout rounded fangs, and sharp pyramidal crowns, with strong basal
ridges separated from the acute blades by deep clefts. Between the
incisors and the canine there is a long diastema.
The _canine_ is of great size; it has a swollen fang, and a long
recurved crown which is much compressed, and has sharp serrated
cutting-edges. The external face is the more convex, the internal has
a well-marked basal ridge, and both are longitudinally striated. The
canine is separated from the premolars by a short diastema.
_Premolars._--The first premolar stands by itself, separated by a
short interval from the second. It is implanted by two fangs, and
has a compressed crown with sharp cutting-edges. It is nearly twice
the size of the corresponding tooth in _Palæosyops_, which is simple
and conical. In the specimen under description there is, besides the
principal lobe of the tooth, a rudimentary anterior lobe which gives
the crown an elongate shape. The basal ridge is shown on the inner
side only. The second premolar has a crown which is oval in section.
The external part consists of two sharp-pointed cusps, separated by
a valley, but confluent at base; while the internal division is a
low ridge (not a pointed cone as in _Palæosyops_) with a tubercle
developed behind it. The basal ridge is marked all around, and sends
up a buttress along the antero-external lobe; as is also the case
in the third and fourth premolars. These are enlarged copies of the
second; but have the internal lobe elevated into a sharp cone, and
showing a much stronger basal ridge and deeper median valley. These
teeth differ in several respects from the premolars of _Palæosyops_.
The crowns are higher and the lobes sharper; the basal ridge is more
distinct, and is not interrupted at the internal cusp; the external
ascending buttress is stronger; and there is no fold between the
external lobes of the fourth.
The molars are constructed in a manner which resembles that of _P.
paludosus_ rather than of _P. major_, but its resemblance to the
form shown in _Titanotherium_ is stronger than to either. They have
broad, square crowns, which increase progressively in size from first
to last. There is a convexity running up the median line of the
external lobes, and the two posterior lobes are connected together
at their bases. The first molar does not exhibit such a decided
increase in size over the last premolar as is seen in _Palæosyops_;
the two external lobes are more erect and sharper than in any form
of that genus. The cingulum is complete even across the internal
cones, and the median valley is very deep, as in _Titanotherium_.
There is a conspicuous fold at the junction of the external lobes,
and a minute tubercle between the two anterior lobes; this tubercle
is present only in the first molar, and not throughout the series,
as in _Palæosyops_. The second molar in general form is like the
first; but is larger, has a deeper median valley, more conspicuous
external folds, and a stronger basal ridge. There is also some change
in the proportions of the crown; the first measuring the same in both
directions, while the second is wider than it is long. The last molar
has but one internal cone; the second is represented by a tubercle;
which however is a true rudimentary lobe and not a development of the
basal ridge. In this tooth, the external fold is very large and the
median valley very deep.
Lower Jaw.--The lower molars and premolars are very much like those
of _Palæosyops_, but are longer, narrower, and higher. The incisors
are not at all like _Palæosyops_, but are compressed and laniariform,
with acute crowns and sharp cutting-edges. The canine is not present
in our specimen; but that it was large, is shown by the long diastema
between the upper canine and incisors.
_Premolars._--The first is small, simple, and implanted by a single
fang close to the canine. The diastema between it and the second
premolar is about one half of that in _P. major_. The second premolar
is the most peculiar tooth in the lower series; as it carries the
development of the anterior lobe at the expense of the posterior
lobe, to a still greater extent than in _Palæosyops_., forming a
crown like the blade of the carnivorous sectorial. The third lobe of
the last molar is no longer a simple cone, but a functional lobe,
with two crests running from it, which bound a median valley.
_Teeth Measurements._
UPPER JAW. M.
Length of entire dental series ·275
Length of incisor series ·375
Length of diastema between canine and incisors ·018
Fore-and-aft diameter of canine ·019
Transverse diameter of canine ·015
Length of diastema between canine and premolars ·0145
Length of premolar series ·083
Length of first premolar ·016
Width of first premolar ·009
Length of second premolar ·021
Width of second premolar ·017
Length of third premolar ·022
Width of third premolar ·022
Length of fourth premolar ·024
Width of fourth premolar ·027
Length of true molar series ·106
Length of first molar ·031
Width of first molar ·031
Length of second molar ·037
Width of second molar ·040
Length of third molar ·038
Width of third molar ·041
LOWER JAW. M.
Length of entire molar series ·202
Length of premolar series ·081
Length of true molar series ·121
Length of second premolar ·024
Width of second premolar ·0125
Length of third premolar ·022
Width of third premolar ·014
Length of fourth premolar ·0235
Width of fourth premolar ·0155
Length of first molar ·030
Width of first molar ·017
Length of second molar ·038
Width of second molar ·021
Length of third molar ·053
Width of third molar ·024
Found at Henry's Fork Divide.
This may eventually prove to be a species of _Telmatherium_, (Marsh);
but the description given by him of that genus is so brief and
uncharacteristic that it might apply to any of the allied genera.
Indeed, Dr. Leidy has regarded it as a synonym of _Palæosyops_.
HYRACHYUS, Leidy.
Proc. Ac. Nat. Sc., 1871, p. 229, _et seq_.
A genus of tapiroid mammals, which is thus defined: "_Lower Jaw:_
Third molar with two crests; four premolars, third and fourth with
one transverse and one longitudinal crest. _Upper Jaw:_ Seven molars,
first without interior heel; premolars with two transverse crests."
(Cope.)
_Synopsis of Species._
I. Long diastema between lower canines and molars.
A. Lower jaw with six molars _H. nanus_.
B. Lower jaw with seven molars.
_a a._ Upper molars with complete cingulum.
Enamel wrinkled.
_a._ The last molar with two external lobes on nearly
the same transverse line _H. intermedius_.
_b._ Last molar with the external lobes on the same
longitudinal line _H. modestus_.
_b b._ Upper molars with cingulum incomplete
externally.
_a._ With anterior conical buttress larger than
anterior lobe.
α. Buttress separate, and no fold from
anterior lobe _H. princeps_.
β. Buttress united, and quite large fold
from anterior lobe _H. imperialis_.
_b._ With anterior conical buttress smaller than
anterior lobe.
α. Tooth large _H. eximius_.
β. Tooth smaller _H. agrarius_.
_c c._ Cingulum only on outer side of first molar,
buttress separated by a ridge from anterior
lobe, fold from this lobe very prominent.
_H. implicatus_.