122 ON THE EXTINCT MAMMALIA OF 



the jaw to be of less depth and more slender proportions than in Merijcochcerus 

 projprius. Below the middle of the last molar the bone is twenty-two lines deep. 



Small fragments of the anterior ends of both jaws, containing fangs of teeth, indi- 

 cate the same arrangement of these organs as in the smallest species. 



In comparing the fossil specimens referred to Merychyxis major with those of 

 Merycoclioerus j^rojmus, they may be observed to be so much alike that, making 

 allowance for difference due to age, or the difference in degree of attrition to which 

 the teeth have been subjected, they may be suspected to belong to the same animal. 

 The anatomical peculiarities already indicated appear to be sufficient to separate the 

 two animals, of which one belongs to the miocene, the other to the pliocene forma- 

 tion. (Compare figures 15, 16, plate XI, with the figures of plate X.) 



LEPTAUCHENIA. 



Leptauchenia is an extinct genus of ruminants founded on some fossil remains dis- 

 covered by Dr. Hayden in 1855, on one of the tributaries of White River, near Eagle 

 Nest Butte, in a formation attributed by him to bed D of the miocene, as indicated in 

 the section page 17. The fossils are more fractured and crushed, and more friable 

 than those of the Oreodons and their associates from the Mauvaises Terres. The 

 matrix is also somewhat different from that of most of the latter, and resembles that 

 attached to the remains of Oreodon major, and approaches that adherent to the 

 remains oi Merycoclioerus. 



The genus is closely allied to Merychyus, so far as can be ascertained from a com- 

 parison of the corresponding parts, and, like the latter, it is related to Oreodon and 

 Merycochoerus. 



The characters observed in the specimens of Lejjtauchenia distinguishing the genus 

 from Merychyus, though apparently slight, nevertheless appeared to me sufficient to 

 separate them, and to be in evidence that other and more striking characters would 

 probably be found in more complete ones. 



The number, relative position, general form and construction of the teeth agree 

 with those of Merychyus. (Compare figures 2 — 5, plate XII, with figures 1 — 5, 

 plate XL) 



The upper true molars have nearly the same form and proportions as in the latter, 

 but the back pair are more uniform in the relation of their transverse and fore and 

 aft diameters, especially in the case of the last one. (Compare figures 5, 8, 12, plate 

 XII, with figures 3, 4, 16, plate XI.) 



The outer buttresses of the crown are relatively more prominent and stronger or 

 thicker, especially the median ones. The latter divide the crown to the fangs more 

 completely than in Merychyus, and their thickened base projects forward so as to fold 



