Established on the left and a portion of the right ramus mandibuli,
containing the first molar.
The jaw presents some differences from the form common in Hyrachyus.
The alveolus is more curved, especially near the ascending portion
of the ramus; the jaw is narrower, and is especially contracted near
the incisive alveolus. The lower margin of the ramus is but slightly
curved. The ramus itself thickens regularly from before backwards,
and is thickest at the last molar; beyond which it becomes thinner
again. The depth of the jaw is comparatively small, and the teeth
have a forward inclination; the mental foramina are not so large as
in other species of _Hyrachyus_, and are four in number.
The incisors are small, arranged in a semi-circle, and have very
much compressed fangs, with somewhat procumbent crowns; the canines
are small, and form a continuous series with the incisors. There
is a diastema between the canine and first premolar; but one which
is shorter than in known species of the genus, and which does not
indicate the usual curve of the upper border of the ramus.
The molar series, (so far as can be judged from their broken
condition), are much the same as in other species; except, perhaps,
in the greater relative size of the posterior lobes, and in the more
complete closing of the median valley by the junction of the two
lobes externally. The ridge which curves downwards and inwards from
the external point of the anterior lobe is unusually well developed,
and curves across the entire crown. The cingulum is feebly shown; it
appears on the anterior and posterior ends of the tooth (first molar)
and for a very short distance on the outside of the tooth opposite
the median valley; there is no trace of it on the inner side.
_Measurements._
M.
Length of entire molar series (about) ·080
Length of premolar series ·033
Length of true molar series (about) ·047
Fore-and-aft diameter of fang of canine ·008
Distance between canine alveoli ·009
Length of diastema ·115
Length of the first molar ·015
Breadth of the first molar ·010
Depth of ramus below last molar ·0245
Depth of ramus below last premolar ·0215
Greatest thickness of ramus ·017
Length of symphysis (about) ·030
Found at Henry's Fork.
Hyrachyus -------- ?
There are three premolars of the upper jaw which we cannot refer
to any known species; but they are too imperfect to justify the
formation of a new species for their reception.
What appears to be a first premolar is inserted by a single broad
fang; it is too much worn to show the characteristic enamel folding,
and is peculiarly broad and short. The second premolar shows the
usual foldings of _Hyrachyus_; save that a distinct valley enters
the tooth from the posterior edge, running forward to the transverse
crest, and inclosed by the curving backwards of the internal cone.
The crown is nearly square; the basal ridge is scarcely marked at all.
_Measurements._
M.
Length of first premolar ·011
Breadth of first premolar ·016
Length of second premolar ·012
Breadth of second premolar ·014
HELALETES, Marsh.
Am. Journ. Sc. and Arts, 1872, p. 218.
Additional material enables us to give further characteristics
of this genus, which may be thus defined: Mammals, allied to
_Lophiodon_, having a third lobe to the last lower molar, short
symphysis, and with the teeth of the lower jaw forming a continuous
series. Astragalus has narrow, oblique condyles.
? - ? 1 - 1 4 - 4 3 - 3
Dental formula: I. -----, C. -----, Pm. -----, M. -----.
3 - 3 1 - 1 4 - 4 3 - 3
Synopsis of Species.
A. Teeth small; a small tubercle on the outer margin,
between the cusps of last upper molar _H. boöps_.
B. Teeth much larger and proportionately broader; no
tubercle on last upper molar _H. latidens_.