DAKOTA AND NEBRASKA. 71 



EUMINANTIA. 



Of this order, the extinct fauna under investigation jiresents the greatest number 

 of species, and by ftir the greatest abundance of materials, indicating the animals 

 to which they belong to have been the most abundant, at least of the larger terres- 

 trial mammals, during the middle and later tertiary periods. The number of species, 

 including some doubtful ones, which I have distinguished, is twenty-seven. These 

 belong to fourteen genera, of which all except one are extinct. Five of the genera 

 appear to represent two peculiar and extinct families; the others belong to the famihes 

 of the Camels, the Musks, and the Deers. About one-half of the species belong to the 

 miocene formations, the remains first appearing in bed A of Dr. Hayden's section, 

 and extending through beds B, C and D. The others belonged to the pliocene period, 

 their remains having been obtained from bed F of Dr. Hayden's section. 



OREODONTID^. 



This peculiar and extinct family of ruminants is distinguished by well-marked 

 characters. The skull has somewhat the form of that of the peccaries ; the cranial 

 portion especially resembles that of the Camel. It is hornless. The temporal fossse 

 are large, and separated by a median sagittal crest, as in the Camel. The zygomatic 

 arches are strong. The orbits are closed behind by an arch. Large and comparar- 

 tively deep fosste impress the lachrymal bones in advance of the orbits. No 

 unossified spaces occupy any part of the face. The auditory capsules are variable in 

 degree of development. The paramastoids are long and strong. The lower jaw is 

 broad and deep posteriorly, and impressed with a comparatively deep fossa below the 

 Imiar notch. The teeth in both jaws form nearly imbroken arches. The fonnula of 

 dentition is : 



3 3 11 4 4 3 3 



Incisors ; c. ; p. m. ; m. =44. 



4 4 11 3 3 3 3 



Well-developed incisors in both jaws; the fourth of the lower jaw bemg a transformed 

 canine, as m ordmary ruminants. Canines well-developed and strong in lx)th ja-\vs, 

 suilline in their resemblance, those of the lower jaw bemg transformed premolars. 

 The anterior three premolars having the crown Lu the form of a demi-cone, with more 

 or less rudimental elements at the base internally. The fourth upper premolars and 

 the true molars of both jaws constructed after the ordinary ruirdnant t^-pe, and most 

 neai'ly resemblmg in fonn those of the Deer family. 



