1881.] '^^^ [Cope. 



digit, is so well developed in HyraeJiyus, 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 Aceratherium, but in America it appears to have 

 been lost earlier. None of the American rhinoceroses of the Lower 

 Miocene of the genus Ccenopus 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 LopModontidce. 



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 vertebrae ; 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 fossse, ahd 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 hj 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 /. infraorMtale. There are two postparietal foramina on one side, and 

 one on the other, above the point of origin of the zygomatic process of the 

 squamosal bone ; and one in the usual posterior position. The post- 

 squamosal has the same anterior position as the anterior postparietals. 



