OREONETES 43 



Oreonetes anceps (Douglass) 1901 

 Fig. 3; PI. I, figs. 6-8; PI. II, figs. 1-2 



Original Reference: Fossil Mammalia of the White River beds of Montana. Trans. Amer. Philos. 

 Soc. (2), XX, pp. 262-264 {Limnenetes anceps). 



Type Localities: On Thompson's Creek, about two miles from the type locality of Limnenetes flatyceps, 

 near Three Forks, Gallatin County (HT), and Big Hole River, north of Dillon, Beaverhead County (PTs), 

 Montana. 



Geologic Horizon: Lower Oligocene (lower Chadron). 



Types: Genoholotype, Cat. No. 745 CM., chiefly a palate, which lacks part of muzzle and right maxilla. 

 Paratypes, Cat. No. 1052 CM., nearly complete skull with dentition; Cat. No. 1118 CM., lower jaw, with 

 teeth but lacking ascending ramus; and Cat. No. 11256 CM., muzzle of a slightly smaller individual. 



Specific Characters: The skull in general has many resemblances to that of Merycoidodon 

 gracilis, except for its somewhat smaller size. It is mesocephalic. The zygomata are nearly parallel 

 to the skull, with the maximum expansion at the anterior part of the glenoid surface. The malar 

 part is moderately deep, and the posterior part of the squamosal portion, which rises above the 

 glenoid surface, is nearly as deep, while the middle area of the arch is much slenderer. The nasals 

 are narrow and somewhat transversely convex. These bones are moderately truncated posteriorly 

 on a line about through the antorbital margin. The pars facialis of the lacrimal bone is small and 

 not semicircular, as it is Limnenetes, while the antorbital fossa is large, deep, and circular in outline 

 and invades the maxillary, as well as occupies a large portion of the lacrimal bone. The frontals 

 are wide and unreduced, extending in advance of the lacrimal bones, and they have a tiny wedge 

 between the posterior ends of the nasal bones. Transversely they are convex along the median 

 suture, then concave, and finally somewhat convex at the supraorbital margin. The frontal plane is 

 flatter and narrower than that of Limnenetes. The orbits are medium in size, smaller than in the 

 latter genus, and they are posteriorly open, with a gape of about 5 mm. They are slightly oval in 

 an anteroposterior diameter. The temporal ridges converge just above the anterior edge of the 

 glenoid surface. The sagittal crest is moderately long but low and is about a fourth of the skull 

 length. The brain case is narrow and elongated. The external auditory meatus is small and 

 directed upward and outward. The basicranial axis is rather steep. The palate is narrow and 

 slightly vaulted, and the U-shaped palatonarial border is opposite the interval between M 2 and M 3 . 

 The glenoid articular surface is anteroposteriorly narrow and concave. The postglenoid process is 

 medium in size, with an oblique external edge. The bullae are very small. The paroccipital 

 processes are moderately small and are in contact with the bulla;. Index: 0.59. 



Mandible: The chin profile is nearly straight, and the symphysiodental angle is 37°. The 

 mental tubercle is present but small. The horizontal ramus gradually increases in depth aft. 



Foramina: The infraorbitals are above the middle of P 3 . The supraorbitals are 12 mm. 

 apart, and on the right side there is a well-marked groove running from the right foramen to the 

 infraorbital foramen. The foramen ovale is large, and there is a medium-sized orifice which I 

 consider to be the foramen rotundum. 



Dentition: The teeth are brachyodont and bear a strong resemblance to those of Mery- 

 coidodon. Superior: There seems to be no trace of the protoconule on the molars such as Proto- 

 reodon shows. P 4 has two small pits in the anteroexternal corner, and the posterior crescent does 

 not reach this corner. P 3 is roughly triangular, with a weak median and anterior intermediate 

 crest and with a well-developed posterior crescent. P 2 is very similarly constructed but is narrower. 

 P 1 is double-rooted, but the crown is too much worn to make it possible to observe the pattern. The 

 canine is large and triangular in section. 



Inferior: The molars closely resemble those of Merycoidodon. The premolars show the 

 posterior crescent united to the median crest, thus forming the moderately large posterior basin. 

 The anterior crescents are undeveloped. 



The superior molar-premolar index is 0.82 and the inferior 0.79. 



