A Skeleton of Great Fossil Beaver, Castorotdes Ohtoensis. 145 
except the anterior pair below, and the posterior pair above, in each 
of which there are four. 
Each three-plated tooth has a deep vertical groove, interiorly, 
in front of the edge of the middle plate, and, exteriorly, behind 
the edges of said plate. These grooves being nearly opposite, give 
the crown of the tooth (or a cross section of the socket), the 
appearance of a depressed hour-glass, leaning to one side and 
having a very large opening from one chamber to the other. 
In a lower series, the anterior and posterior teeth have about 
the same relative width across the crowns. In an upper series, 
the crown of the first molar is decidedly the widest, and they 
gradually narrow backward. ‘The following cuts give the natural 
size of the right lower and left upper series, and the relative posi- 
tions of the enamel, dentine and cement. 
G C D 
z fe 5 S 

o.... 
D xe ae 
Owey 
Fie. 5.—Natural size. Molars of left upper side: a, anterior; p, posterior; c, 
cement; d, dentine; e, enamel. 

Mle 
D Cc 
Fic. §.—Natural size. Molars of left upper side: a, Anterior: p. posterior; ¢, 
cement; d, dentine; e, enamel rings. 
As the ridges and depressions on the worn crowns run diagon- 
ally across, averaging an angle of about forty-five degrees, it gives 
the teeth the appearance of being twisted about one-eighth of an 
entire revolution. From the inner margins of the crowns the lines 
of fold extend outward and forward, both above and below. 
. The folds, as seen in the cuts, are not uniform as to thickness, 
