144 Cincinnati Society of Natural History. 
generic distinctions. In /zber and Arctomys the lines of the series 
antero-posteriorly are nearly parallel. In Castor they converge 
rapidly forward. In Castoroides they converge still more rapidly, 
the distances anteriorly and posteriorly in the upper series being as 
one to three. As in. Castor, the upper grinders slant backward and 
outward, while the lower slant forward and slightly inward. The 
leaning inward from a median line is less marked than in Caséor. 
The greatest length of the upper series along the crowns is 2.62 
inches; length of lower series, 2.75 inches; length of anterior 
upper molar, 1.88 inches; length of posterior upper molar, 1.75 
inches; length of anterior lower molar, 1.94 inches; length of 
posterior lower molar, .go inch; antero-posterior diameter of crown 
of anterior upper molar, .62 inch; lateral diameter of crown of 
anterior upper molar, .65 inch; antero-posterior diameter of crown 
of posterior upper molar, .76 inch; lateral diameter of crown of 
posterior upper molar, .52 inch. 
The grinders are made up of flattened enamel tubes enclosing 
plates of dentine. Said enamel tubes are held together by cement, 
otherwise they would be entirely distinct from each other from 
crown to base. While the disposition is to cleave into slabs on 
the decay of the cement, as in the grinders of the Mammoth, yet 
there is no common union of parts at the base as in the latter, nor 
is there anything approaching the nature of fangs. 
They are set in the socket as a tenon in a mortise, showing the 
same unique folding at base as at crown. The flattened enamel 
tubes are hollow at base. 


Fic. 4.--Natural size. Base of second lower left nolan showing entire absence of 
fangs, and also how the plated structure extends from crown to base. 
For illustration, here is the middle enamel tube in the base of 
the second lower molar of the left side. It is a slit, less than a 
line wide and nearly three-fourths of an inch long, running diag- 
onally across the base. A medium-sized needle probes these tubes 
from one-fourth to one-half an inch. 
Each grinder is composed of three flattened enamel tubes, 


