jgg THE MERYCOIDODONTIM- 



Pronomotherium siouense Sinclair 1915 

 Figs. 120-123 



Original Reference: Additions to the fauna of the Lower Pliocene Snake Creek beds. Proc. Amer. 

 Philos. Soc, LIV, pp. 86-87, fig. 11. 



Type Locality: Sinclair Draw, T. 25 N., R. 55 W., Sec. 3 (SE %, middle), Nebraska. Collecting 



locality 1000C. 



Geologic Horizon: Upper Miocene (lower Snake Creek, Horizon A). 



Types: Holotype, Cat. No. 12057 P.U.M., right half of lower jaws, practically complete, except for con- 

 siderable wear on crowns of molar-premolar series, collected in 1914. Plesiotype, Cat. No. 18333 A.M.N.H., 

 well-preserved skull, collected in 1921 in Sinclair Draw, Quarry B. (Matthew, 1924 A, defines this as a neotype, 

 implying that the holotype has been lost. Such is not the case here, and I consider the specimen in the plesiotype 

 class.) 



Specific Characters: The plesiotype skull is little smaller than P. laticefs but much smaller 

 than P. altiramus. It is more mesocephalic than any of the skulls of the other species. The form 

 of the muzzle is intermediate between these two. The maximum expansion of the zygomata is at 

 the anterior of the glenoid surfaces. The malar is deep beneath the orbits, but the arches are light 



Fig. 1 20. —Pronomotherium siouense Sinclair. Skull. PLT. Cat. No. 18333 A.M.N.H. 2/5 nat. size. 



(Photograph from the American Museum.) 



and nearly parallel with the tooth row. In shape they are intermediate between the two above- 

 mentioned species. The nasal bones are missing, but from the position of the nasofrontal sutures it is 

 evident that these bones were small and ascended sharply upward and forward, as in P. laticefs. 

 The recession of the nares seems to be less extreme than in the latter species and the skull appears to 

 be flatter, while the upper margins of the maxillaries and premaxillaries are less convex and shorter. 

 The lacrimal bone apparently has very little or no pars facialis, but the sutures are not clearly dis- 

 cernible. The antorbital fossa is of the same general pattern but is relatively smaller than in 

 P. altiramus or in P. laticefs. The frontals are flat, following the generic pattern. The orbits are 

 elevated and oval, with the greater diameter extending upward and backward. The temporal ridges 

 unite on a line above the posterior part of the glenoid processes to form a short, thin sagittal crest. 

 The supraoccipital crest overhangs the occipital condyles much less than in the other species, and 

 the wings are wide apart. 



The brain case is small and typical. The palate is narrow and somewhat concave. The 

 palatonarial border is situated well behind the last molars, farther aft than in the other two species. 

 The basicranial axis is very steep and the basicranial region very much foreshortened, so that the 

 paroccipital processes, bullse, and postglenoid processes are greatly crowded together. The glenoid 

 surface is moderately convex anteroposteriorly, and the postglenoid process is transversely short and 

 not markedly dependent. The bulla is of very small diameter and extends downward as an 



