450 MONOGKAPUS OF NOltTLI AMERICAN RODENTIA. 



same relative proportions and form as in (he latter animal." Tlic 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 Eucastor, they are closed loops, such as are seen in 

 a transverse section of the tooth of Castor* 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 Eucastor, 

 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 Eucastor, 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 the middle one, 

 which, instead of being transverse, is placed somewhat obliquely. The sec- 

 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 Eucastor. 



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 tor/us. 

 The differences in dentition between Castor and Eucastor are, however, as 

 great as between Castor and Trogontherium. Doubtless, other correspond- 



" See Owen's Odontography, i>lute cvi, lig. 2. 



