226 THE MERYCOIDODONTID/E 



Dentition: Owing to the tight closure of the mouth, little opportunity is afforded for study of 

 the dentition. The teeth, however, are not crowded, and there is no particular shortening of the pre- 

 molars. Externally they do not show any characters unusual to Merychyus. 



Discussion: This species, although of upper Rosebud age, represents what we should expect 

 the primitive Merychyus to be. The premolar series is only a trifle shortened, the molar-premolar 

 ratio being 0.80, whereas in Merycoidodon it is 0.83 and in Merychyus elegant 0.71. Apparently 

 this form had no facial vacuities, although, so far as known, all the other species possessed them. 

 This also is an ancestral character, as are the lack of crowding of the premolars and their almost 

 complete lack of shortening in the anterior parts of P 2 and P 3 . Perhaps this ancestral form persisted 

 in South Dakota as a backwash from the main evolutionary stream which was advancing in Nebraska 

 and Wyoming, with the culmination in the former state. 



Merychyus elegans Leidy 1858 

 PI. XXXI, figs. 5-8 



Original Reference: Notice of remains of extinct Vertebrata. Proc. Acad. Nat. Sci. Phila., X, p. 25. 



Type Locality: Niobrara River, Nebraska, between the mouth of Turtle River and a point 300 miles 

 upstream. 



Geologic Horizon: Upper Miocene (Bed F of Hayden). Hayden (1858, p. 149) defines his Bed F as 

 follows: 



1st, dark gray or brown sand, loose, incoherent, with remains of Mastodon, Elephant, etc.; 2nd, sand and 

 gravel, incoherent; 3rd, yellowish white grit, with many calcareous, arenaceous concretions; 4th, gray sand with 

 a greenish tinge; contains the greater part of the organic remains; 5th, deep yellowish red arenaceous marl; 

 6th, yellowish gray grit, sometimes quite calcareous, with numerous layers of concretionary limestone from two to 

 six inches in thickness, containing fresh water and land-shells. 



Types: Genocotypes, Cat. Nos. 120 and 121 U.S.N.M., halves of upper and lower jaws with teeth. From 

 the evidence available these specimens seem to have been collected in the fourth horizon. 



Specific Characters: Complete skulls and jaws are not known at present, but from measure- 

 ments of the dental series and from the character of contiguous parts, the species is apparently about 

 the size of Leptauchenia major, or perhaps a trifle smaller than M. arenarum. The skull is short- 

 ened, with the premaxillaries coossified. 



Mandible: This has a nearly straight inferior border, increasing in depth aft beneath the tooth 

 row and with no marked angle. The symphysis is fairly strong, sloping downward and backward at 

 approximately an angle of 40° with the dental series. 



Foramina: The infraorbitals lie above P 4 . 



Dentition: The teeth are hypsodont. The superior incisors are small and cylindrical. The 

 canines are moderately large. The anterior two premolars appear to slope backward. These, 

 together with P 3 , are anteriorly much reduced. All the teeth are offset at contiguous surfaces, except 

 P 3 and P 4 whose contiguous surfaces meet in the same plane. The molars increase in length rather 

 rapidly and uniformly from 1 to 3. In the earlier forms this increase is much more gradual. The 

 parastyles and mesostyles, faint on M 1 , increase in prominence on the after molars. The meta- 

 style on M 3 is moderately well developed. The molar-premolar index is 0.71, the same as in 

 M. siouxensis. 



The inferior dental series are crowded. The incisors are small, and I 3 overlaps C, which is 

 about twice the size of the former. Pi is strong, longer than wide, and separated from P 2 and C by 

 minute diastemata. P 1} P 2 , and P 3 overlap. P 4 is well developed posteriorly. The molars increase 

 in size from 1 to 3, and M 3 is twice as long as Ml The metastylid of M 3 is directed somewhat 

 inward from the plane of the faces of the paraconid and metaconid. 



