RODENTIA—CASTORINAE-CASTOROIDES. 
363 
CASTOROIDES, Foster. 
Molars , rootless ; incisors grooved longitudinally on the outside, consisting of thin laminae of dentine encircled by 
closed rings of enamel, united by cement. Of these rings there are three in each tooth, except the first upper and last lower, 
which have four. 
CASTOROIDES OHIOENSIS, Foster. 
Castoroides ohioensis, J. W. Foster, 2d Report Geology of Ohio, 1838, 81 ; fig. 
Hall, Bost. Jour. N. H. V, 1847, 385. (Geol. Position.) 
Wyman, Bost. Jour. Nat. Hist. Osteology, Y, 1847,391,401. 
I have thought proper to introduce a brief notice of this gigantic North American fossil 
mammal, both on account of its close relationship to the beavers and its great size, which con¬ 
siderably exceeds that of the Capybara, the largest of living rodents. In the intimate struc¬ 
ture of the teeth it hears quite a resemblance to this animal, although totally different in other 
respects. Of any peculiarities of structure other than those of the skull we are thus far ignorant, 
in the entire absence of other remains. 
Very few remains of Castoroides have hitherto been discovered; the best known specimens 
being those of a lower jaw from Ohio first brought to notice by Mr. Foster, and the nearly 
perfect skull obtained by Rev. Benjamin Hale, of Geneva College, and upon which was founded 
the able monograph of Messrs. Hall and Wyman, in the Journal of the Boston Society, as 
quoted above. 
The following description of the principal characteristics of the skull of Castoroides , as com¬ 
pared with that of the beaver, is based chiefly on the excellent plates of the memoir above 
referred to. 
The general form of the skull, as seen from above, is much as in Castor. The cranium is, 
however, much shorter, narrower, and less capacious, and the distance from the posterior margin 
of the zygomatic process of the temporal bone to the back part of the skull proportionally much 
less. The nasal bones preserve somewhat the same proportions, reaching, possibly, not quite 
so far back, but instead of occupying exactly one-third of the total length of the upper outline 
of the skull, take up more nearly two-fifths of the same length. The malar bone is much less 
massive, as for five-sevenths of its total axial length it is quite narrow, the upper and 
lower edges nearly parallel beyond the temporal bone, and curving gently upwards; after which 
it widens, with the sides again nearly parallel, and bending very rapidly upwards. Amteriorly, 
too, it is not prolonged so as to occupy the anterior external edge of the orbit and articulate 
with the lachrymal, but the upper maxillary constitutes altogether the anterior root of the 
zygoma, and forms the anterior wall of the orbit. The zygoma itself is situated much higher 
up on the side of the head than in Castor; its lower edge being but little more than half-way 
between the top of the head and the grinding surfaces of the molars, while in the beaver it is 
much nearer the latter. The lower edge of the anterior root of the zygoma being thrown so 
much higher up, the ante-orbital foramen ascends also, being midway on the side of the head, 
instead of much nearer the lower surface ; this foramen appears no larger proportionally than 
in Castor , and is similarly placed anterior to the root of the zygoma; it is, however, more 
visible from the side, not being concealed by a bony plate. 
The lachrymal bone is very small, and separates the frontal and maxillary bones in the ante- 
