232 THE MERYCOIDODONTID^; 



rounded, thus differing from the usual form in this genus. The malar is shallow and stout, with a 

 truncate inferior edge. The squamosal process invades the malar to a point below the posterior 

 fourth of the orbit. The wide frontals are nearly flat. The temporal ridges unite at an acute angle 

 just back of the postorbital constriction to form a barely perceptible sagittal crest up to a point 

 above the posttympanic process, whence the crest gradually rises, thus giving a saddleback profile, 

 which is another unusual feature. The supraoccipital crest does not overhang the condyles, and the 

 wings are wide apart. 



The brain case is full, and the sides are strongly convex. The external auditory meatus is large 

 and extends outward and backward but apparently not much upward. The palate was probably 

 nearly flat, and the V-shaped palatonarial border seems to have been about on a line with the pos- 

 terior of M 3 . The inferior part of the type skull has been much damaged. The inflated bulke are 

 longer anteroposteriorly, the anterior border being on a line with the front of the postglenoid process 

 and the posterior border being bounded by the paroccipital process. The postglenoid process is 

 robust and not compressed to the extent that is average in Merychyus. The paroccipital process is 

 elongate and acuminate, becoming compressed inferiorly so that its greater diameter is fore and aft. 

 Index: 0.51. This index is just on the borderline between a dolichocephalic and mesocephalic type 

 of skull. 



Mandible: Posteriorly the border is regularly convex, beginning just below the condyle. The 

 angle is marked. The ramus decreases rapidly in anterior depth. The coronoid process is decidedly 

 small, with the sigmoid notch not excavated below the level of the condyle, and the latter is slightly 

 convex upward. The masseteric fossa lies above the line of M 2 and is not anywhere sharply 

 demarcated. 



Foramina: The infraorbital foramina lie above the anterior border of P 4 . The supraorbitals 

 are 8 mm. from the mid-line, with grooves leading forward from them. 



Dentition: In the type, only M 2 and M 3 on the left side are undamaged, and in the paratype, 

 an old individual, the crowns are worn. The two molars seem to correspond closely to the compar- 

 able ones in M. arenarum, both in size and in crown pattern. The styles are perhaps somewhat more 

 prominent than normal in certain of the other species of Merychyus. Apparently the premolar 

 series was unusually long, as inferred from the paratype, and made a molar-premolar index of 0.89, 

 the highest in the genus. 



Discussion: Cope was uncertain about the generic position of this species, suggesting that it 

 might belong in Merycochcerus or in Eucrotaphus (= Eporeodon). As we now define these genera, 

 it is clear that both can be ruled out of consideration. 



For the present I shall consider the species as referable to Merychyus but keep an open mind. 

 In most of the characters now known it is nearest to that genus, but the long brain case, the slender, 

 low, and angulate zygomatic arch, the presumably larger size, the superior skull contour, and the 

 high molar-premolar index are all at least specific differences from the normal seen in Merychyus. 

 In comparison with Ticholeptus the differences are still greater. 



Merychyus siouxensis Loomis 1924 

 Fig. 169; PI. XXXIV, figs. 5-6 



Original Reference: Miocene oreodonts in the American Museum. Bull. Amer. Mus. Nat. Hist., LI, 

 Art. 1, pp. 31, 33, fig. 21. 



Type Locality: Ten miles west of Agate, Sioux County, Nebraska. 



Geologic Horizon: Lower Miocene (lower Harrison). 



Type: Holotype, Cat. No. 13774 A.M.N.H., well-preserved skull. 



Specific Characters: This skull is about the size of that of M. elegans and M. e. paniensis. 

 The zygomata are slender and short, while the malar is rather heavy below the orbit. The zygomatic 



