TRIPLOPUS. 679 



tirely rudimental character of the fifth metacarpal, which, with its digit, is 

 so well developed in Hyracliyus, 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 lophiodonts with the rhinoceroses. The fourth 

 digit (the fifth) was retained by the earliest type of rhinoceros in Europe, 

 the genus AceratJieriwn, but in America it appears to have been lost earlier. 

 None of the American rhinoceroses of the Lower Miocene, of the genus 

 Coenopus 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 Rhinocerontidce, and with the character of the feet place it between 

 that family and the LophiodontidoB. 



As yet but one species of Triplopus is certainly known, but a second 

 is placed in it provisionally. 



Teiplopus cubitalis Cope. 



American Naturalist, 1880, p. 383. Proceedings Amer. Philos. Soc, 1881, p. 383. 

 Plates LVa, figs. 10-12 : LVIo. 



This species is represented by a nearly entire skuH with lower jaw; 

 most of the cervical vertebrae; a left anterior limb nearly complete; a part 

 of the left scapula, and a part of the right anterior limb ; all belonging 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 interorb- 

 ital 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 fossse, and at 

 the posterior premaxillary teeth. The top of the muzzle narrows rapidly 



