1881.] : 383 | Cope. 
digit, is so well developed in Hyrachyus, places Triplopus in another 
family, and in another line of descent. I think that it must be regarded 
as one of the forms of the series connecting the tapirs with the rhinoceroses. 
The fourth digit (the fifth) was retained by the earliest type of rhinoceros 
in Europe, the genus Acerathertum, but in America it appears to have 
been lost earlier. None of the American rhinoceroses of the Lower 
Miocene of the genus Canopus Cope present it, and in the present genus 
we have an ancestral type of the Eocene period, in which the last digit is 
already lost. The premolars of different structure from the true molars, 
exclude this genus from the Rhinocerontida, and with the character of the 
feet place it between that family and the Lophiodontida. 
As yet, but one species of Triplopus is certainly known, but a second is 
placed in it provisionally. 
TRIPLOPUS CUBITALIS Cope. 
American Naturalist, 1880, p. 383. 
This species is represented by a nearly entire skull with lower jaw ; 
most of the cervical vertebre ; a left anterior limb nearly complete; a 
part of the left scapula, and a part of the right anterior limb ; all belong- 
ing to one animal. The specimen was not quite adult, as the last superior 
molar is just protruding its crown through the maxillary wall, and the 
last two superior milk premolars still remain in place, much worn and 
closely pressed by the overlying successional teeth. 
The cranium is peculiar in its wide orbital region, and short compressed 
muzzle ; the latter is damaged in the specimen so that the form of the 
nasal] bones cannot be determined, except at their proximal portions. The 
interorbital space is plane in both directions, and rises very gently 
posteriorly. The sagittal crest is narrow and low, until above the meatus 
auditorius, where it rises. Above the posttympanic process it bifurcates, 
and each rounded lateral lobe extends posteriorly to a point above the 
occipital condyles. Viewed from above the head is wide between the 
zygomatic fosse, and at the posterior premaxillary teeth. The top of the 
muzzle narrows rapidly above the latter, but does not contract below until 
the first premolar is reached. The zygomatic arch is not convex along its 
middle, and encloses a narrow fossa. The superciliary border is prominent, 
and nearly straight, and is bounded by a notch behind. The squamoso- 
occipital ridge is well marked. The posttympanic process is shorter than 
the paroccipital, and is separated from it by an open shallow groove, 
which is probably bottomed by the mastoid bone. The paroccipital pro- 
cess is much narrowed below and is turned a little outwards. There are 
two closely adjacent tubercles on the anterior border of the orbit, probably 
on the lachrymal bone. 
Foramina. Only a few of these are well preserved ; among the lost is 
the f. infraorbitale. There are two postparietal foramina on one side, and 
one on the other, above the point of origin of the zygomatic process of the 
squamosa} bone; and one in the usual posterior position. The post- 
squamosal has the same anterior position as the anterior postparietals, 
