MERYCHYUS 



225 



This species is primitive in several characters. The antorbital fossa is small ; the molars do not 

 increase markedly in size from 1 to 3 and are unusually broad ; the skull is slightly more brachy- 

 cephalic (index 0.62, the same as in M. delicatus) ; the anterior parts of the premolars, while some- 

 what reduced, have not reached the stage of development seen in M. elegans, and their total relative 

 length is a trifle longer (index 0.74). This form reminds one of a male with the broader skull, 

 heavier skeleton, and wider, shorter molars, but there are sufficient characters by which at present it 

 can be differentiated from the other species. I should term it one of the primitive merychyids, with 

 M. delicatus, a female, as one of the more primitive. 



Merychyus delicatus Loomis 1924 

 Fig. 164 ; PI. XXXII, figs. 9-10 



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

 Art. 1, pp. 31, 33-34, fig. 22. 



Type Locality: Three miles northeast of Porcupine Butte, Porcupine Creek, South Dakota. 



Geologic Horizon: Lower Miocene (upper Rosebud). 



Type: Holotype, Cat. No. 12980 A.M.N.H., skull, lacking posterior part, and jaws, still joined by matrix, 

 together with anterior cervicals. 



Specific Characters: The species is the smallest so far described and is of light build. The 

 zygomata are missing from the skull. Apparently it has no facial vacuity. The nasals are rounded 

 posteriorly and are widest just posterior to the narial opening. They are nearly flat in both direc- 



Fic. 164. — Merychyus delicatus Loomis. Skull and jaw. HT. Cat. No. 1 2980 A.M.N. H. 1/2 nat. size. 



(After Loomis, 1924.) 



tions. The lacrimal bone is of moderate size, and the lacrimal fossa is very shallow and faintly 

 defined. The frontals appear to be fairly flat, but they have been crushed down to some extent. 

 The orbits are elevated, more as in Leptauchenia, and are nearly circular. The temporal ridges 

 unite in advance of the glenoid surfaces to form a short sagittal crest. The brain case is short but 

 wide. The postglenoid processes are small, the bullse very large, and the paroccipital processes 

 slender, extending only a little way below the bullse. Index: 0.62a. 



Mandible: The chin is rather steep, forming an angle of 53° with the tooth row, and vertically 

 it is very slightly concave. Its lower edge is nearly straight, with the angle ill defined. The coro- 

 noid process is short and thin, the sigmoid notch shallow and U-shaped, and the condyle nearly flat. 



Foramina: These cannot be defined. 



