28 
PROCEEDINGS OP THE ACADEMY OF 
ginally discovered by Dr. Hayden on Bijou Hill. Numerous teeth of tlie 
species are contained also in tlie Niobrara collection. 
PACHYDERMATA. 
Rhinoceros crassus, Leidy. 
The Niobrara collection contains small fragments of two lower jaws of young 
animals, a much worn upper incisor, a last upper molar, and an upper pre- 
molar, apparently of the deciduous series, of a species of Rhinoceros, which 
appears to have had almost the same size and formula of dentition as the recent 
Indian Rhinoceros, (R. indicus.) 
One of the fragments of lower jaws, consisting of the symphysial portion 
with sockets for four incisors, indicates these to have had the same relation of 
size and form as in R. indicus. 
The upper lateral incisor has nearly the form and size of the corresponding 
tooth of R. indicus. The antero-posterior diameter of its crown is two and 
one-third inches, and its transverse diameter three-fourths of an inch. 
The last upper molar, which belonged to an old individual as indicated by 
its worn condition, is of the form usual in most species of living Rhinoceros. 
Its antero-posterior diameter internally is two inches, and its oblique dia- 
meter posteriorly about half an inch more. 
The upper deciduous premolar has the narrower portion of its crown ante- 
riorly. The triturating surface of the specimen presents a tract of dentine on the 
outer wall and summits of the inner lobes of the tooth. The outer wall has a 
median ridge externally, corresponding to the most elevated point of its triturat- 
ing surface. The anterior border of the outer wall has the same form as the 
posterior one, but is shorter and more prominent. The inner lobes are em- 
braced by a strong basal ridge, as in R. occidentalis and Aceratheriiim incisivum. 
The antero-internal lobe curves inward and backward, and the succeeding lobe 
is transverse. Three conspicuous vallies bound the lobes, of which the middle 
one is deepest and the anterior one least so. From the outer wall of the tooth 
two folds project into the middle valley, and the posterior of these join 
one springing from the anterior face of the postero-internal lobe, thus isolat- 
ing a deep pit from the valley. If the fossil tooth were worn away to a level 
with its basal ridge it would exhibit four distinct enamel pits ; one corres- 
ponding to the outer end of the anterior valley, two for the middle valley, and 
one for the posterior valley. 
Antero-posterior diameter of the tooth externally, 19 lines. 
Transverse diameter of the tooth posteriorly, 19 " 
Transverse diameter of the tooth anteriorly, 16 " 
Mastodon (Tetralophodon) mirificus, Leidy. 
One of the most interesting discoveries of Dr. Hayden in the pliocene depo- 
sit of the valley of the Niobrara, is the greater portion of the lower jaw of a 
Mastodon, most undoubtedly distinct from that species whose remains are so 
abundantly found in later deposits in the United States. The specimen be- 
longed to an old individual, as the last molar tooth had protruded and is con- 
siderably worn off at its anterior two-thirds ; and it indicates a much smaller 
animal than the M. ohioticus. The form of the jaw is like that of the existing 
Elephant of India. A single tooth, the last molar, occupies each side of it, and 
resembles the corresponding one of M. angustidens, of Europe, or of M. sivalen- 
sis, of the Sivalic Hills, of India. The crown measures nine inches antero-poste- 
riorly, and three and a half inches transversely, and possesses six transverse 
rows of conical lobes closely crowded. In advance of the sixth molars, pre- 
served in the specimen, no traces of alveoli are left for preceding teeth, but a 
sharp sigmoid ridge extends to the front of the jaw. 
The greatest breadth of the jaw outside the position 
of the molars is 15 inches. 
Length from back of last molar to anterior end of 
the jaw, 16 
Length of the sigmoid ridge in advance of the molars, 9 ' ' 
[March, 
