Oope.] : 544 : [Sept. 20. 
becoming first obtuse, and then disappearing. Posteriorly the reverse 
process takes place, and proportions increase. But in the last molars 
they do not assume the proportions seen in Palwotherium and allied 
forms. They increase in the elevation of corresponding ridges of the 
crescents, and decrease in the others, so that the resultant form is 
nearly like that of Dinothertwm or perhaps Lophiodon. The outer ridge 
of one crescent appears as a cingulum, which sinks to the base of the 
crown from the apex. This is rudimentalin the genera just mentioned. 
The corresponding bounding ridge of the other crescent is reduced to a 
rudiment extending diagonally across the valley between the remaining 
crests, as is seen in not a few genera of the Eocene. 
We have thus an explanation of the heretofore obscure question as to 
the origin of the crescent-bearing tooth of the Artiodactyles. From the 
two crested type of Tapirus, the two-angled form developes itself by the 
growth of the cingulum and diagonal crest just described. This is seen 
completed in Palwosyops. The elevation of the ridges and deepening of 
the intervening valleys, would result in the ordinary Ruminant type. 
The same process increasing transverse crests only, derives the Mastodont 
from the Tapiroid form, and the deepening of the valleys of this, again 
results in Hlephas. 
In comparison with Bathmodon semicinctus, Cope, the crowns of the 
premolars are of similar size, but considerably less elevated. 
The measurements cannot be given with exactitude, but are approxi- 
mately as follows: Superior incisor crown, width. 75 inch; elevation .60 
inch. Canine 1.25 inches from apex, inner face .75 inch. Premolar 
length. 76, width 1.1 inch. Molar length crown 1.1 inch, width 1.25 inch. 
Inferior premolar, length of crown 1 inch, width .75 inch. Posterior mo- 
lar, length 1.30 inch, width .9 inch. The crests of the last mentioned are 
quite elevated, one more than the other; the lower with a strong cingu- 
Jum at the base, which rises to what is homologous with the base of a tri- 
angle, or outwards; none on the inner aspect of the base of the crown. 
The cingula of the superior molars are only anterior and posterior. 
This large ungulate was found in a stratum below those of the Green 
River Group of Hayden, or in the lower beds of that series, near Black 
Buttes, Wyoming. Obtained by the Geological Survey under direction of 
Dr. F. V. Hayden. 
In a line of banks or low blufts, immediately below that in which the 
Metalophodon was found, dermal scutes of a small crocodilian are abun- 
dant. The discovery of the greater part of a cranium of one of these 
enables me to point out the existence of a species of Alligator of still 
smaller size than the smallest of the Caimans at present inhabiting 
South America, This species, which I call ALLIGATOR HETERODON, 
possess several peculiarities. The anterior and posterior teeth differ 
exceedingly in shape ; the former are flattened, sharp-edged, and slightly 
incurved ; the edges not serrate. Those of the premaxillary bone are 
subequal in size, while one behind the middle of the maxillary is larger 
