182 
EIGHTH ANNUAL REPORT. 
remains of an extinct horse and mastodon, in the Monograph on Rodentia 
(vol. XI) of the United States Geological Survey of the Territories, by F. V. 
Hayden. It seems, therefore, to have been extended over the whole of the 
United States east of the ^Mississippi from INIinnesota to Louisiana, and into 
Texas, and to have been cotemporary with the mastodon, and hence with 
the mound builders. It was, however, quite different from the living 
beaver, and may not have been acquatic. No portions of the skeleton 
except the head and teeth have been discovered. Its size was about that of 
the common ^Ipx?^ bear, according to Mr. Allen, and it was wholly a vege- 
tarian. ^ 
The spooimens ^fonnd at Minneapolis consist of the left ramus and the lower 
left incisor, the latter evidently broken from the former in being removed 
from the sand in which the whole was entombed. Their size indicates an 
animal somewhat larger than the specimen first found in Ohio and described 
by Foster, and also larger than that found in New York. It is, however, a 
little smaller than that described by Wyman from Memphis. The whole 
length of the specimen, when the parts are united, is 9)^ inches, of which 
inches consist of the projecting, uncovered incisor, a portion of the jaw 
being broken away on the under side. The condyle and coronoid process 
are wanting, and the sigmoid notch is also gone. On the under portion of 
the mandible the alveolar cavity of the incisor is broken into between the s}tii- 
physis and the angular (?) process, disclosing the dark-brown enamel of the 
incisor. The angular process is about half an inch in length, directed 
obliquely inward and backward Its base extends antero-posteriorly an inch 
and a half. Its shape is that of a blunt rounded wedge, and its under surface 
is in a plane at right angles to the grinding surface of the molars. The four 
molars are all preserved perfectly. The first one, which rises a little more 
than half an inch in front, above the alveolar cavity, has four, obliquely 
tran.sverse lamellae, or flattened hollow plates, covered with enamel and 
cemented together, one after the other, by layers of crusta petrosa, v^hich 
also seems to fill their interior. Within the alveolar cavity these plates, or 
sacks, at least in the fourth molar, are separate and free, and when this 
tooth is taken out their lower ends are open. The outer surfaces are finely 
striated perpendicularly, and crossed transversely by undulations of growth. 
The second and third molars have each three lamelke, the first and last of 
which are obliquely transverse but parallel, while the second is more oblique- 
ly transverse and longer, nearly touching the interior angle of the third and 
the exterior angle of the first. The lamellae all cross the mandible from 
within obliquely outward and forward. The second and third molars are of 
nearly the same size and shape, but they rise less above the alveolar cavity. 
The}' are sunk deep within the mandible, along the outside of the incisor. 
The enamel ridges on the grinding surfaces form a broad letter S. In 
the fourth molar the dentinal plates are three in number and more nearly 
parallel, and less oblique to the general direction of the grinding surface. 
These plates terminate on the upper surface of the incisor, which passes 
below, or along the inside of the bases of all of the molars. The symphysis 
of the mandible, where it united with the other ramus, is three inches long, 
there being a thickening of the bone and a downward process on the under 
side of the ramus where the incisor in use would most need a powerful ful- 
crum. The greatest diameter of the incisor, where broken, is one inch. 
