450 MONGGRAPHS OF NORTH AMERICAN RODENTIA. 
same relative proportions and form as in the latter animal.” The molar teeth, 
however, differ very much in structure from those of Castor, more resembling 
in some respects those of Trogontherium. In Castor, the loops of enamel are 
all open externally; while, in Ewcastor, they are closed loops, such as are seen in 
a transverse section of the tooth of Caséor.* In Castor, the molars decrease 
very gradually in size posteriorly ; the first being but little wider, though con- 
siderably longer than the second, the third slightly smaller in both diameters 
than the second, and the fourth still smaller. In Trogontherium, the first 
three molars, in form and relative size, greatly resemble those of Fucaséor, 
and, in the pattern of the grinding surface, there is a closer resemblance than 
there is to the same teeth in Castor; but, in Trogontherium, the third molar, 
according to Owen, is smaller than the fourth. In Euwcastor, the first molar 
is equal in length to the length of both the second and third, and propor- 
tionately wider. It has three closed loops of enamel; the anterior and pos- 
terior loops being each rather less than one-half the size of tie middle one, 
which, instead of being transverse, is placed somewhat obliquely. The see- 
ond and third teeth have each but two, the anterior of which is, in each 
tooth, about twice the size of the posterior one. The teeth are also set more 
obliquely in the jaw, and the first molar has a deep infolding of enamel near 
the middle of the inner margin of the tooth, directed obliquely forward. ‘In 
all these features, there is a closer resemblance to Trogontherium than to 
Castor. 
The specimen described by Dr. Leidy belonged to a “quite aged 
individual”, and to this fact he deemed were possibly due the differences in 
the structure of the molars seen in this form as compared with Castor, as he 
had not at the time an opportunity of comparing his ‘specimen with equally 
aged examples of the common Beaver. A large series of the skulls of the 
American Beaver now before me, embracing those of all ages, show that the 
permanent molars differ very little in form, or in the pattern of the triturating 
surfaces, with age. A transverse section of the tooth of Castor, however, 
displays much the same structure as is seen in Lucastor. 
As shown by the above-cited references, Dr. Leidy at first referred this 
form to a new subgenus of Castor, but later cites it simply as Castor tortus. 
The differences in dentition between Castor and Eucastor are, however, as 
great as between Castor and Trogontherium. Doubtless, other correspond- 
* See Owen’s Odontography, plate evi, fig. 2. 
