PERISSODAGTYLA. 
321 
HIPPOTHERIUM, Kaup. 
Hipparion, Christ. 
Hippotherium calamarium, Cope. 
Plate Ixxv, figs. 1 - 2 . 
Add. Eept. Chief of Engineers, 1875, ii, p. 990. 
This three-toed . Horse is indicated by the oral and palatine parts of 
the skull with the superior dental series of both sides, together with one 
mandibular ramus, with all its teeth, of an individual from near San Ilde- 
fonso, and also probably by molar teeth of two individuals from the Loup 
Fork beds of Colorado. The species is allied to the H. paniense^ Cope, and 
differs from the S. occidentale, H. speciosum, and H. gratum of Leidy, in the 
relative form and size of the internal anterior dentinal column. As in the 
species first named, this column is subcylindric, and equal to, or smaller 
than, the posterior internal columnar fold. In the three species last named, 
the anterior column is flattened or oval in section, and often larger tlian 
the posterior columnar fold, and submedian in position. 
In the typical or New Mexican specimen, the anterior column is large, 
and its center is anterior to the middle transverse line of the crown. In the 
present state of attrition, which has left two-thirds of the crowns of the 
median molars, this column presents an angalar projection toward the inner 
anterior crescent, betraying an approach to the union seen in Protoliippus., 
which is in the fifth molar of the right side of the present Horse actually 
accomplished through the medium of a narrow isthmus. The dentinal band 
connecting the inner crescents throws out two, rarely one, folds toward this 
column. The borders of the lakes are much plicate; the posterior border 
of the anterior lake having from four to six inflections. The posterior 
border of the posterior lake exhibits one deep inflection, which is generally 
bifurcate. 
The first premolar is quite small, and is two-rooted. The second is a 
wide tooth, different in form from the elongate corresponding tooth of the 
H. occidentale, its anterior lobe being but little prominent. The palate is 
wide and well arched. The mandibular teeth are elongate, but not narrow; 
and the anterior lobes are well developed, especially the median ones. No 
basal cingula on teeth of either jaw. The last molar is smaller than the 
three preceding it, and the anterior lobe of the first is quite narrow. 
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