218 



the merycoidodontim: 



Mandible: This has a straight inferior border. The strong, fused symphysis makes a 35° angle 

 with the tooth row. 



Foramina: The infraorbital lies above the posterior of P 3 . The supraorbitals are about one- 

 third of the distance from the median line to the supraorbital borders. 



Dentition: See under M. elegans for comparison of teeth. 



Fie. 160. — Merychyus arenarum (Cope). 



Skull and jaw, partly restored. HT. 

 nat. size. (After Cope, 1888.) 



Cat. No. 8146 A.M.N.H. About 1/2 



Discussion: This species has a wider muzzle and forehead than has M. a. leptorhynchus, and 

 the canines are more robust. It is a trifle more brachycephalic (index 0.63) than M. siouxensis 

 (index 0.57), but the molar-premolar index is nearly the same, being 0.72 in M. a. leptorhynchus 

 and 0.71 in M. siouxensis. 



Merychyus arenarum leptorhynchus Cope 1884 



PI. XXXII, figs. 4-8 



Original Reference: Synopsis of the species of Oreodontids. Proc. Amer. Philos. Soc, XXI, pp. 536, 

 537-540 {M. arenarum subspecies leptorhynchus). 



Type Locality: East of Laramie Peak, Wyoming. 



Geologic Horizon: Lower Miocene (upper Harrison). 



Type: Holotype, Cat. No. 8149 A.M.N.H., nearly complete skull with jaws. Collected by J. C. Isaac. 



Specific Characters: The skull is a little longer than those of M. curtus and M. a. minimus. 

 It is mesocephalic, in fact almost dolichocephalic, the index (0.50) being on the borderline between 

 the two. The superior contour is gently arched. The zygomata are slender, reaching their maxi- 

 mum expansion somewhat in advance of the glenoid surface. The posterior part rises gently, and the 

 anterior process invades the malar to a point beneath the posterior third of the orbit. The malar is 

 deep beneath the orbit. The facial vacuity is rather small and more than twice as long horizontally 

 as vertically. It is bounded mainly by the maxillary, in part by the lacrimal, and probably to some 

 extent by the frontal, but this last is not capable of positive proof, owing to the crushing in this 

 particular area. The nasal bones are flat, rather narrow, with the posterior borders rounded. 

 Posteriorly they terminate well in advance of a line across the anterior orbital borders. The lacrimal 

 bone is large, having a greater diameter anteroposteriorly than vertically, and forms about half of 

 the lower border of the vacuity. The lacrimal fossse are large but shallow. The frontals are nearly 

 flat in both directions, moderately wide, and extend forward on either side of the nasal bones as well 



