1884. ] 561 [Cope. 
riochoerus trifrons sp. nov. 
This species is known to me by a single cranium of an immature indi- 
vidual. It lacks of perfection only the basioccipital, the pterygoid, and 
the alveolar border of the premaxillary bones. It retains the third and 
fourth deciduous premolars, while the third true molar is still in its alveo- 
lus, where it is exposed in place. 
Although the specimen is immature, its characters will not permit me to 
place it with any other species known to me. I have specimens of like age 
of the A. guyotianus, and these are quite different. From A. ryderanus 
it differs in the form of its otic bulla, ete. 
The muzzle and front form a flat horizontal profile, while the pariétal re- 
gion is convex. The profile descends gently to the supraoccipital border, 
or inion. The muzzle is compressed above and below the canine alveolus, 
and there is a concavity above the third and fourth premolars, and behind 
the foramen infraorbitale above this fossa the lachrymal region is con- 
vex. The nasal bones are lost, so that the form, of their posterior suture 
cannot be ascertained. The frontal bones are gently concave in transverse 
section between two lines produced forwards from the anterior extremities 
of the temporal ridges, that is at the postorbital constriction of the cranium. 
These lines are represented by a rounded longitudinal angle, from which 
the frontal bone descends to the superciliary border on each side. A trace 
of this form is seen the A. ryderanus. The supraorbital borders diverge 
outwards and backwards to the postorbital processes. These are prominent 
horizontally, and are abruptly decurved at the apex. The temporal ridges 
enclose an urceolate area, having a gentle convexity in their direction be- 
fore they unite at a point more posterior than in the other species, that is 
above a line connecting the anterior borders of the postglenoid processes. 
The malar bone is slightly concave on the external face, and is mode- 
rately deep, and not thick. The squamosal part of the zygoma is rather 
slender, and does not rise above the postglenoid process. Its superior edge 
continues without interruption into the posterior temporal crests, and so 
into the supraoccipital. The, postglenoid process is like that of A. guyo- 
tianus, narrow and produced downwards. Paroccipital lost. The otic 
bulla is large, its anterior edge extending anterior to the postglenoid pro- 
cess. It is nearly twice as large as in JZ guyotianus, and extends much 
further forwards. It presents two flat sides, one external, the other out- 
wards and forwards, and a convex side inwards and backwards. These 
sides meet at an angular edge below, which runs outwards and backwards. 
The sphenoid bone is convex between the bulle. Basioccipital lost. The 
palatonareal border is convex, and is opposite the middle of the second 
true molar. In the mature skull it would be probably more posterior. The 
palate is everywhere concave in transverse section. 
The frontopariétal suture is broadly convex, and is opposite the anterior 
edge of the glenoid surface, and 25 mm. in advance of the sagittal crest. 
The anterior processes of the bone on each side of the nasals are wide and 
truncate, and do not extend beyond the interior suture of the lachryma) 
