266 
PERISSODACTYLA. 
and the angle of the outer descends to form an anterior basal cingulum.’ 
There is no trace of the inner posterior cusp which is seen in the genus 
Orotherium, External cingulum scarcely a trace. Enamel nearly smooth. 
The most instructive specimen includes both maxillary bones with six 
molars, and fragments of the mandibles with the posterior three molars. 
The second premolar has two external cusps, without ridges on the outer face, 
and the inner face slopes inward, without internal cusp or lobe; the base 
is a little longer than wide, that of the other molars wider than long. On 
the true molars, there is a strong continuous external cingulum, but that on 
the inner side is only present between the lobes and as a faint fold on the 
base of the anterior. The external cusps are strongly convex on their outer 
faces, but are not ridged, nor does any ridge interrupt the deep valley 
between them, as in the H. procyoninum. The intermediate cusps are dis¬ 
tinct, but weak; the enamel is nearly smooth. The penultimate molar is 
the largest. In the lower molars, the external cingulum is well marked, 
and there is none on the inner side. The last molar is rather shorter, and 
the ramus a little deeper than in the specimen first described. 
Measurements. 
Length of the last three inferior molars.. 
Length of the last inferior molar... 
Width of the last lower molar... 
Depth of the ramus at the first molar. 
Length of six superior molars... 
Length of the second premolar. 
Width of the same. 
Length of the third premolar.. . 
Width of the same. 
Length of the penultimate molar. 
Width of the same. ..... 
Length of the last molar. 
Width of the same.. .. 
M. 
0. 022 
0. 009 
0. 0055 
0. 0155 
0. 0380 
0. 0055 
0. 0045 
0. 0048 
0. 0065 
0. 0071 
0. 0088 
0. 0070 
0. 0071 
Some of the specimens now referred to this species I formerly supposed 
might belong to the H. procyoninum * but the discovery of the superior 
molars of the II. angustidens has enabled me to make a direct comparison, 
which reveals marked distinctions. 
* Eyracotherkm procyoninum, Cope.— Helothermm proeyoninum, Cope, Proc. Ainer. 
Philos. Soc., 1872, p. 466 (August 3); ? Orohippus pumilus, Marsh, Amer. Journ. Sci. 
and Arts, 1872 (August 7). 
