194 the merycoidodontim: 



Skeleton: The skull is rather large in proportion to the remainder of the skeleton. The body 

 and limbs are of medium weight and length. Promerycochcerus had short, heavy limbs and body, 

 whereas in Merychyus we find a slender, long-limbed form of skeleton. The limbs and feet are 

 more like those of Eporeodon and Mesoreodon, from which Ticholeftus may well have descended. 

 There are four toes on manus and pes. 



Ticholeptus rusticus (Leidy) 1870 

 Figs. 142-143; PI. XXVIII, figs. 1-2; PI. XLVII, fig. 1 



Original Reference: Remarks on a collection of fossils from the Western Territories. Proc. Acad. 

 Nat. Sci., Phila., XXII, p. 109 (Merycochcerus rusticus). 



Synonym: Brachycrus rusticus Matthew 1901. 



Type Locality: Sweetwater River, Wyoming; northeastern Colorado. 



Geologic Horizon: Upper Miocene (Pawnee Creek). 



Types: Holotype, Cat. No. 145 U.S.N.M., part of skull and lower jaw of left side containing complete 

 dentition. Paratypes, all in U.S.N.M., Cat. Nos. 442, 532, 535, 536, 537, and 913, various teeth and bones. 

 The above types are figured in Leidy 1869, on plates 3, 7, and 20, and are all from the same locality. Plesio- 

 types, Cat. No. 19-104 A.M., nearly complete skeleton; Cat. No. 9050 A.M.N.H., incomplete skull and jaws, 

 with greater part of skeleton; Cat. No. 9056 A.M.N.H., pair of complete lower jaws; Cat. No. 9115a 

 A.M.N.H., pair of lower jaws, one incomplete, and three superior molars; and Cat. No. 9049 A.M.N.H., fore 

 leg and foot, with fragments of various parts of skeleton. 



Specific Characters: The skull is longer than that of T. brachymelis and is perhaps about 

 the size of that of T. bannackensis, although a complete skull of the latter is not known. The whole 

 skull is lightly built. The zygomata reach the maximum expansion just anterior to the glenoid 

 surface. The malar is deep, but the squamosal portion of the arch is lighter than that of any related 

 form and does not thicken posteriorly but continues as a thin plate into the lambdoidal crest. The 

 facial vacuity is large and is intruded into the lacrimal, maxillary, and frontal bones. The nasal 

 bones are much reduced, with their forward termination over P 3 , and their truncated posterior ends 

 about on a line through the antorbital margin. They are flat anteroposteriorly and follow the nearly 

 straight upper contour of the skull, while transversely they are convex. The lacrimal bone has a 

 large -pars facialis, and the lacrimal fossa is shallow. The frontals are not reduced, and are moder- 

 ately flat medially but distinctly convex above the orbits. They do not carry forward so far as the 

 -pars facialis. The premaxillaries are small and medially coossified. The orbit is moderately small 

 and is located farther aft than is that of either T. brachymelis or T. breviceps. The postorbital bar 

 is decidedly heavy. The temporal ridges are not prominent and extend backward in a gentle sweep 

 to unite over the glenoid surfaces. The sagittal crest is short, light, and low. The supraoccipital 

 crest is not produced far beyond the occipital condyles, and the wings are widespread. The brain 

 case is more elongate than is usual in Ticholeptus and is not widely expanded. The external 

 auditory meatus opens obliquely upward but is not placed nearly so high as in Merycochoerus. The 

 basicranial axis is rather steep, the bullse are small and not inflated, and the mastoid plate is but 

 moderately expanded. The glenoid surface is nearly flat, and the postglenoid process is antero- 

 posteriorly thin, although it is unusually deep. The paroccipital process is moderately robust and 

 long. Index: 0.57-0.62. 



Mandible: The symphysis is strong, and the chin is straighter than in many species of this 

 family, with a symphysiodental angle of nearly 60°. There is a noticeable tubercle at the inferior 

 part. The inferior border of the ramus is nearly straight, and the angle is not prominent. The 

 ascending ramus is deep and wide, with the condyle set well inward from the posterior margin. The 

 coronoid process is short and rather stout, the sigmoid notch shallow and wide open, and the 

 condyle low. 



