APPENDIX. 



Art. XXX.~Principal Characters of the Tilhdontia : by 0. C. 

 Marsh. (With two plates.) 



The Eocene deposits of North America have yielded two 

 new orders of extinct mammals, the Dinocerata, and the TUlo- 

 dontia, both of great interest, and widely different from all 

 known groups, as well as from each other. The latter order, 

 recently established by the writer,* is comparatively little 

 known, as the animals representing it are of moderate size, and 

 but few of their remains have vet been discovered. The typ- 

 ical genus of this order is TillotheHum, the more important 

 characters of which can now be readily determined from speci- 

 mens in the Yale Museum. This genus, therefore, will be 

 mamly used in the present article to illustrate the order. 

 Tillotherium Marsh, ISTS.f 



The skull in this genus resembles in its general form that of 

 Ursus. It is of moderate length, much elevated in the frontal 

 region, and with the zygomatic arches widely expanded. 

 (Plate VIII.) The posterior portion of the cranium is de- 

 pressed, and much constricted behind the fronto-parietal suture. 

 The temporal fossae are large, and separated by an obtuse 

 sagittal crest. There is no postorbital process. The frontal 

 bones are large, and inflated with air cavities. The nasals are 

 elongate, broad posteriorly, and narrow in front, where they 

 unite with the premaxillaries. The latter are massive, and pro- 

 ject forward beyond the nasals. They are united only by a 

 slender bridge of bone, below the anterior narial aperture. 



The orbit is confluent with the temporal fossa, which is 

 largely formed below by the squamosal The latter sends out- 

 ward and forward a strong zygomatic process, and, downward, 

 a short, obtuse, post-glenoid tubercle, which bounds in front the 

 external auditory meatus. This opening is bounded behind by 

 the posttympanic process of the squamosal, which unites di- 

 rectly with the paroccipital. The tympanic portion of the 

 periotic does not reach the external surface. The articular face 

 for the condyle of the lower jaw is but very slightly concave, 

 (i'late IX) The malar bone is slender, and forms the anterior 



*This Journal, vol ii, p. 221, March, I8T5. f Vol r, p. 485. 



A.M. Jour. Set.— Third Series, Vol. XI, No. 63.— March, 1876 



