A Skeleton of Great Fossil Beaver, Castoroides Ohtoensis. 143 
alveolus, and they are more than an eighth of an inch apart at their 
extremities. In this specimen they are one-fourth of an inch 
apart, and slightly converge till they touch at extremities. The 
longitudinally ribbed and fluted character of the incisors is here 
exhibited to perfection, as are also the numerous delicate wrinkles 
extending across the grooves. The number of longitudinal grooves, 
from the anterior inner angle round to the posterior sinus, is 
nineteen. 
The beveled crowns are 1.37 inches in the antero-posterior diam- 
eter and one inch across, laterally. This, it will be noted, does not 
give the relative diameters of the teeth, since the crowns are worn 
obliquely. The circumference of these incisors taken together, two 
inches from the extremity, is 5.5 inches; circumference of one of 
them, 3.25 inches; lateral diameter of one, .96 inch; antero- 
posterior diameter, .85 inch. 
They are covered with enamel anteriorly, on the distal surfaces, 
and as far around on the lingual side as the fluted surface extends. 
The proximal surfaces are slightly concave antero-posteriorly, and 
the lingual surface has a well-marked, shallow sinus, with the greater 
bounding ridge interiorly. There appears to be no enamel on the 
proximal, or on the adjoining surfaces of the lingual, sides. The 
lower incisors are 10.75 inches in length. They project 5.25 
inches. 
The proximal and lingual sides are flattish, with each a shallow 
longitudinal sinus, while the antero-distal side is convex, and has 
the fluted surface corresponding to the upper incisors, there being 
thirteen grooves near the crown. 
In gnawing, while the upper incisors barely touched each other 
at their extremities, the inferiors must have powerfully pressed 
together and sustained each other, as their proximal surfaces, near 
their extremities, have worn each otheraway equal to one-fourth their 
lateral diameter, so as entirely to obliterate the mesial sinus for an 
inch or more. As these teeth approach each other in the two 
rami, and are crowded from their original direction by pressing 
together, their growth would be spiral if they could grow without 
wearing away at the crowns. I have before me an isolated left 
lower incisor which is clearly spiral. The antero-posterior diam- 
eter of these incisors, midway their free portion, is .87 of an inch; 
lateral diameter .75 of an inch. 
The grinders are well known to be of special interest as affording 
