XN 
ON THE OSTEOLOGY OF THE EXTINCT TAPIROID 
HYRACHYUS. 
By Pror. E. D. Corr, A. M. 
(Read before the American Philosophical Society, April 18, 1878.) 
This genus was originally described by Leidy* from portions of skele- 
tons of individuals from the Eocene tertiary of Wyoming. He recognized 
it as related to the Lophiodon of Cuvier in dentition, and as sharing with 
characters of that Eocene genus, peculiarities which belong to the exist- 
ing genus Tapirus. 
Having obtained a large series of remains of this genus, including 
more or less numerous portions of six species with nearly complete 
skeleton of H. eximius, Leidy, I propose to give such an account of its 
osteology as will place its relations on a certain basis. 
The characters which distinguish its dentition from those of the allied 
genera are as follows: 
Tapirus, Briss. Lower jaw: third molar two- crested ; three premolars, 
the third and fourth with two transverse crests. Upper jaw: seven 
molars, first with an inner heel tubercle; other premolars with two 
transverse crests. 
Hyrachyus, Leidy. Lower jaw: third molar with two crests; four pre- 
molars, third and fourth with one transverse and one longitudinal crest. 
Upper jaw: seven molars, first without interior heel; premolars with 
two transverse crests. 
Lophiodon, Cuvier. Lower jaw: third molar with three cross-crests ; 
premolars three, Nos. 2 and 8 with longitudinal crests. Upper jaw: pre- 
molars with longitudinal crest only ; No. 4 with two transverse crests. 
Upper jaw: premolars with only one transverse crest. 
In Hyrachyus the nasal bones are elongate, and unite with the maxil- 
laries anterior to the orbit; in H. extmius above the foramen infra-orbitale 
exterius ; in Zapirus those bones are much shortened, and either do not 
unite with the maxillaries or join them and the frontals above the orbit 
at different points from the anterior to the posterior borders. The tem- 
poral fosse are so extended as to produce an elevated sagittal crest, 
which is bifurcate behind, each projection continuing along the outer 
margin of the occipital region as a lateral crest. The tympanic bone is 
unossified beneath the meatus auditorius externus, which is bounded in 
‘front by astrong postglenoid process. Posteriorly it is bounded by along 
descending mastoid process of the squamosal bone, nearly closing it 
below. This is bounded posteriorly by a long and stout paramastoid pro- 
cess, which is compressed from before backwards and curves backwards 
and inwards. The foramen magnum has prominent supero-lateral mar- 
gins which are néarly straight, and unite at a right angle above. 
The dentition is thus: I. $; C.4+; P.M. 4 M. 3; a considerable dia- 
stema separates the premolars and the canine. 
In the species studied, the vértebre are divided as follows: C. 7; D. 
* Hayden’s U. 8. Geological Survey of Montana, 1871, p. 361. 
