322 
PERISSODACTYLA. 
The molars of one of the specimens from Colorado are closely similar 
in all respects, except that the anterior lobe of the second premolar is a 
little more produced. I also refer here the tooth described as “ M. 2 ” 
under H. jjaniense, in Eeport of the United States Geological Survey of the 
Territories, 1873, pp. 522-523. As compared with the molars of that species, 
those of H. calamarium differ in the much greater complexity of the enamel 
folds; those of 11. paniense being the simplest in the genus, even more so 
than in H. affine. The anterior inner column of H. paniense lacks the char¬ 
acteristic inner angle seen in the present Horse. 
Length of the seven superior molars... 0.134 
Length of the three true molars.. 0. 062 
Length of the second premolar... 0.026 
Width of the crown of the second premolar.... 0. 020 
Greatest length of the second premolar.. — .. ' . 0.022 
Greatest width of the second premolar-. . 0. 024 
Greatest length of the penultimate molars. 0. 022 
Greatest width of the penultimate molars. 0. 022 
Width of the palate (chord) between the middles of the third premolars- 0. 042 
Length of the crown of the antepenultimate lower molar.,. 0. 021 
Width of the crown of the antepenultimate lower molar... - 0. 012 
The typical specimen belongs to an adult animal, and was taken from 
the matrix by myself without admixture of others. 
Hippotherium speciosum, Leidy. 
Plate Ixxv, tig. 3. 
Extinct Mammalia of Dakota and Nebraska, p. 282. 
A series of superior molars nearly identical in character with those orig¬ 
inally described as typical of this species, by Dr. Leidy, and figured on 
plate xviii, figs. 16, 18, 19, of the work above quoted, and agreeing with 
specimens from Colorado, was obtained near Pojuaque. Some of the speci¬ 
mens described by Dr. Leidy as of doubtful reference under the head of this 
species obviously do not belong to it. 
TROTOHIPPUS, Leidy. 
Leidy, Extinct Mamm. Fauna Dakota and Nebraska, 1869, p. 275.—Gope, Ann. Eeport 
U. S. Geol. Surv. Terrs., 1873, pp. 519-527. 
This genus, originally described by Leidy from dental characters, I 
