DAKOTA AND NEBRASKA. 71 



RUMINANTIA. 



Of this order, the extinct fixuna under investigation presents the greatest numljer 

 of species, and by far 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 beloiag to the families 

 of the Camels, the Musks, and the Deers. Aljout 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. 



OREODONTIDjE. 



This peculiar and extinct family of ruminants is distinguished hj well-mai"ked 

 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 fossae 

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

 arches are strong. The orbits are closed behind jjy an arch. Large and com2:)arar 

 tively deep fossae impress the lachrymal bones in ad\'ance 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 postei'iorly, and impressed mth a comparatively deep fossa below the 

 lunar notch. The teeth m both jaws form nearly unbroken arches. The formula of 

 dentition is : 



33 11 44 33 



Incisors : c. ; \). m. ; m. =44. 



4 4 11 3 3 3 3 



Well-developed incisors in both jaws ; the fourth of the lower jaw being a transfonned 

 canine, as in ordinary ruminants. Canines well-developed and sti'ong m both jaws, 

 suilline in their I'esemblance, those of the lower jaw being transformed premolars. 

 The anterior three premolars having the crown in the form of a demi-cone, vnth. more 

 or less rudimental elements at the base internally. The fourth uj^per premolars and 

 the true molars of both jaws constructed after the ordinary ruminant tj'pe, and most 

 nearly resembling in form those of the Deer fiimily. 



