DAKOTA AND NEBRASKA. 123 



over the surface of the lobe in advance, instead of expanding into the base of the 

 crown as in the Last-named genus. The interv^enitig outer faces of the external lobes 

 are more oblique, or are directed more outward and backward than in Meryclujim. 

 (Compare figures 4, 6, 7, 11, plate XIl, with figures 1, 15, plate XI.) 



The interspaces separating the inner and outer lobes of the upper true molars of 

 Leptauchenia appear, in the unworn condition of the teeth, even to have been pi'opor- 

 tionately less capacious than in those oi Meri/cJii/u>i. From their gaping mouths they 

 more rapidly narrowed, so as to be reduced to a fine vertical fissure approaching the 

 bottoms of the crescentic pits. In consequence of this arrangement, from attrition of 

 the teeth, the gaping mouths of the pits early disappeared, leaving the molars with 

 broad surfaces of exposed dentine bordered by enamel, and provided each with a pair 

 of median crescentic enamel bars, bounding an almost imperceptible trace of a fissure, 

 the remnant of the interlobular spaces. Figures 5, 8, 12, plate XII. 



The inferior true molars likewise closely resemble those of Meri/chyus. As in this, 

 they are also remarkable for the early obliteration of the interspaces of the inner and 

 outer lobes. (See figures 3, 14, 17, 20.) From those of Merychyus they usually 

 diflTer in the decided separation internally of the inner lobes, by means of a narrow 

 fold or well-defined and slightly overlapping ridge extending to the bottom of the 

 crown. (Compare figures 16, 19, plate XII, with figures 7, 13, plate XI.) 



Some additional and more complete remains of Xeptai<c7ie«ia, including those of 

 another species, were obtained by Dr. Hayden in his expedition of the summer of 

 1866, on White Earth Creek, a tributary of White River. The fossils and pertaining 

 matrix have the same general appearance as those from Eagle Nest Butte, and like 

 them were derived from bed D of the miocene, as indicated on pages 17 and 20. 



The fossil specimens oi Leptauchenia, though far less well preserved than those of 

 Merychyus, are more complete in extent, though more mutilated and crushed. 



Tlie cranium oi Leptauchenia has the general form and construction of that of 

 Oreodon. Large temporal fossiB are separated by a well-developed sagittal crest. 

 The auditory bulla3 are of enormous proportions. The forehead is broad and nearly 

 flat. The orbits are closed by a post-orbital arch. The infra-orbital arches are 

 strong. Lachrymal fossa? exist in front of the orbits. The infra-orbital foramina are 

 situated above the third premolars. 



Large unossified spaces exist in advance of the frontals above the fore part of the 

 orbits, extending forward, over and in advance of the lachrymals. These unossified 

 spaces, proportionately much larger but similar to those existing in the Deer, Lama, 

 and some other living ruminants, are entirely absent in Oreodon. The fossils referred 

 to Merycocheerus and Merychyus do not retain the part of the skull in which the un- 

 ossified spaces are present in the specimens of Leptauchenia, so that their existence in 

 the former remains a question to be. solved by future discovery. 



