MERYCHYUS 219 



as between them and the maxillaries. The orbits are rounded subquadrate, with the anteroinferior 

 angle somewhat produced. They are medium in size and not elevated. The temporal ridges are 

 well marked but not prominent and unite above the anterior of the glenoid surfaces to form a moder- 

 ately long sagittal crest, which has a gently convex rising profile. The supraoccipital crests do not 

 overhang the condyles, and the wings are wide apart. The brain case is long and narrow and not 

 laterally expanded. The external auditory meatus is broken away. The skull has been somewhat 

 compressed laterally, but the palate probably was narrow, proportionate with the whole skull, and 

 the V-shaped palatonarial border was apparently on a line with the last tooth. The basicranial axis 

 is shallow. The glenoid surface is plane transversely and slightly convex anteroposteriorly, rising 

 aft. The postglenoid process is compressed and rather elongate and is wider transversely than is 

 usual in this genus. The paroccipital process is long and flat posteriorly. Index: 0.50. 



Mandible: The chin forms a 40° angle with the tooth row and is moderately concave vertically. 

 The horizontal ramus rapidly narrows anteriorly, and the angle scarcely descends below the inferior 

 border. The ascending ramus is rather wide, the coronoid process short and thin transversely, the 

 sigmoid notch wide open, and the condyle nearly flat and anterosuperiorly directed. 



Foramina: The infraorbital is double on the right side, one orifice being above the extreme 

 posterior part of P 3 and the other above the interval between that tooth and P 4 . The supraorbitals 

 are small, lie about a third of the distance between the mid-line and the supraorbital border, and have 

 grooves leading from them almost straight forward to the outer edges of the nasal bones. The 

 foramina of the basicranial region cannot be definitely made out. 



Dentition: Superior: incisors small; canine quite small ; first three premolars much shortened 

 anteriorly; P 1 set obliquely. The premolars, as well as the molars, follow the Merychyus pattern. 

 Inferior: three incisors small; Pi slender, with narrow crown and acute apex; first three premolars 

 obliquely set and overlapping; P 4 and molars crowded, following usual pattern. 



Discussion: Cope described this as a subspecies of M. arenarum. In physical features it is 

 exceedingly close to that species. M. a. le-ptorhynchus, however, has a shorter and narrower skull 

 (index 0.50) and a relatively longer premolar series, making the molar-premolar index 0.77, 

 whereas the same indices for M. arenarum are 0.63 and 0.72 respectively. The lower jaw is 

 slenderer and of shallower depth anteriorly. 



Both species are from the same horizon and locality, and I believe it is possible that M. a. lepto- 

 rhynchus is a female of the more robust form. The alternative supposition is that we have a 

 slenderer but otherwise quite similar subspecies or dwarf variety living with more brachycephalic and 

 somewhat stouter individuals. The differences between these two forms seem to me to be doubtful as 

 of full specific value. 



Merychyus arenarum minimus Peterson 1906 

 Figs. 6, 161; PI. XXXIII 



Original Reference: The Miocene beds of western Nebraska and eastern Wyoming and their vertebrate 

 fauna. Ann. Carnegie Mus., IV, pp. 56, 67-68, fig. 16 (Merychyus minimus subsp. nov.). 



Type Locality: Sioux County, Nebraska. 



Geologic Horizon: Lower Miocene (upper Harrison). 



Types: This species is based on a series of fifteen individuals, of which the holotype is Cat. No. 1466 CM., 

 consisting of the skull, mandibles, fragments of vertebra, limbs, and a manus, and the plesiotypes are Cat. Nos. 

 1331, 1403, 1439, 1462, 1525, 3852, and 3853 CM., consisting of various skeletal parts used in the description 

 and in illustrations of the skeleton. 



Specific Characters: The skull is a little larger than M. curtus and a little smaller than 

 M . a. leptorhynckus. The zygomatic processes of the squamosals are in part broken away, but the 

 anterior portions invade the malars to a point below the posterior third of the orbit. The malar is 

 moderately heavy and deep. The facial vacuities are large and oval in shape, with the maximum 



