558 



A R C H ^E T II E HI U i^I OF NEBRASKA. 



these teeth are subjected, the anterior valleys commence to be obliterated irom 

 their termination. 



In the second to the fourth molars inclusive, the anterior of the inner lobes is the 

 larger, and all the latter become dilated just before their termination, and confluent 

 from their bases, most so in the third molar, and least in the fourth. The anterior 

 and posterior valleys are nearly of the same depth at their termination externally, 

 except in the fourth premolar, in which the posterior valley is more shallow. The 

 basal ridge of these teeth is well developed all round, envelopes the base of the 

 postero-internal lobe to twice the extent it does that in advance, and from its great 

 vertical extent posteriorly adds very much to the depth of the posterior valley. 



The first molar still retains the two internal lobes, of which the posterior is very 

 distinct, but the anterior is reduced to the condition of a tubercle rising out of the 

 basal ridge, and connected by a transverse lamina with the internal side of the 

 outer portion of the tooth. 



The inferior molars (Tab. xii. b, 5; xv. 3), six in number on each side, closely 

 resemble those of Acerotherium incisivum. They possess a basal cingulum developed 

 all round. The first of the series in the outline of a transverse section forms an 

 isosceles triangle, but like all the others, it is constituted of two lobes, of which the 

 anterior is so compressed laterally as almost wholly to lose its crescentic appearance. . 



ADMEASUREMENTS. 



Length from occipital condyle to anterior part of first molar, 

 Greatest breadth at zygoma;, .... 



Breadth at infra-orbitar foramina, .... 



" just above root of zygoma;, 

 Length of molar scries, ..... 

 Autero-posterior diameter of the sixtli upper mohir, 

 Transverse " '•' '' " " 



Antero-posterior diameter of the lifth lower molar, . 

 Transverse '< " « <' " . 



Inches. 



Miles. 



9 



2 



5 



9 



2 



3 



2 



8 



4 



6 





11 





11 





lOJ 





6 



ARCIIyEOTHERIUM. Leidy. 



(Tab. X., figs. 1, 2, 3; Tab. xi., fig. 1.) 



Archacotherium Mortoni, Leidy: Pr. Ac. N. S., vol. v., p. 92. 



This remarkable Pachyderm, judging from the form of its teeth, is closely allied 

 to the Choeropotamus, Cuvier, and the Ilyracotherium, B. Owen. 



The species was first indicated by a fragment of the face containing the posterior 

 two premolars of the left side. In Dr. Owen's collection is a more characteristic 

 specimen, consisting of the middle portion of the face, much mutilated, of A. Mortoni. 

 This fossil contains the first and second true molars on both sides entire, excepting 

 the former of the right side, which has a portion broken off. It also possesses the 

 alveoli still retaining the fangs of the last molars, and also those for the last pre- 

 molars, and a portion of those in advance. 



