94 ON THE EXTINCT MAMMALIA OF 



Oreodon gbacilis. 



Remains of Oreodon gracilis, the smallest discovered species of the genus, occur 

 much less frequently than those of 0. Calbertsoni, probably not because the animal 

 was formerly less abundant, but from the fact that the remains of smaller animals 

 are more liable to the usual causes of destruction than those of larger ones. 



The skull of Oreodon gracilis is on the average about two-thirds the size of that of 

 0. GLdhertsoni, and it has the same general form, proportions and construction, but 

 differs in a few details. Different skulls of the sjDecies present considerable variation, 

 as in the case of 0. Gidhertsoni. The usual size and form of skull is represented in 

 the fine specimen of figures 2, 3, plate VI. 



The principal distinctive characters of the skull of Oreodon gracilis compared with 

 that of 0. Oulbertsoni, independent of the difference of size, are as follows : 



The sides of the cranium are relatively more convex. The sagittal crest usually 

 expands just in advance of the middle of its course into a triangle, which is rarely 

 the case in 0. Guliertsoni. The face or snout at its upper part is usually broader in 

 relation with its size and is also less convex. The nasal bones are relatively wider, 

 and their posterior extremities together, received between the frontal angular pro- 

 cesses, usually form the half of an ellipse in outline. The supra^orbital foramina are 

 absolutely much more removed from each other. The orbits are more rotund or the 

 transverse diameter more nearly approaches the vertical, and occasionally even 

 slightly exceeds it. The lachrymal fossae are relatively more shallow, and are usually 

 confined to the lachrymal bones. The auditory bullaB are relatively much larger, 

 exceeding in size the contiguous jDost-glenoid tubercles. 



All the distinctive characters are subject to modification, and in some specimens go 

 so far as to assume the condition of those of the corresponding parts of 0. Gulhertso^ii. 



Among the many specimens of skulls of Oreodon gracilis I have had the oppor- 

 tunity of examining, the following have been selected as exhibiting the peculiarities 

 and variations of the species : 



1. An almost complete skull, represented in figures 2, 3, plate VI. The sagittal 

 crest begins to expand about the middle of its course and forms a narrow triangular 

 extension to the anterior deeply notched border of the parietal. The upper part of 

 the face is rather narrower than usual in the species and more convex. The fore- 

 head is about as convex as is usual in 0. Gidbertsoni. The supra-orbital foramina 

 include a space of half an inch. The posterior extremities of the nasal bones to- 

 gether form the half of an ellipse, seven lines long in the median line and eight and 

 three-quarter lines wide between the frontal angular processes. The nasal bones are 



