332 ON THE EXTINCT MAMMALIA OF 



vaticns. They vary among themselves in size and robustness, but independently of 

 what may be considered as individual peculiarities, the differences appear to depend 

 mainly on difference of age. 



In regard to the form of the jaw fragments, consisting merely of the portions con- 

 taining the molar teeth, they agree nearly with the corresponding portions in the 

 Rabbit, except that they appear proportionately rather more robust. In a corres- 

 ponding position of the lower jaw there does not exist, in the fossils, the reticular 

 foramina observed in the vicinity of the mental foramen in the Rabbit. 



Three specimens, consisting of portions of as many upper jaws, contain molar 

 teeth, as represented in figures 14 — 16. plate XXVI, magnified three diameters. The 

 teeth of the different specimens exhibit considerable variation in size, form, and ap- 

 parent constitution ; nevertheless they are suspected all to have belonged to the 

 same species, and the differences are supposed to be due to difference of age. 



The teeth of the youngest specimen, figure 14, are the smallest of the three series, 

 and consist of all the molars except the last one, part of the alveolus of which is also 

 retained by the specimen. The anterior three teeth belong to the deciduous set, and 

 are inserted into the jaw by firngs, or at least an unenamelled portion of the crown. 



The first tooth of the series is the smallest, and resembles that of the Rabbit. The 

 crown is divided by an anterior valley into a pair of lateral lobes. The succeeding 

 two temporary teeth have their triturating surface bordered by enamel, except on the 

 outer part. The second tooth exhibits a short internal valley, and a small central 

 islet. The fourth molar tooth, almost as large as that in advance, is bordered by 

 enamel except externally. It possesses an internal transverse fold or valley and a 

 central islet. The fifth molar, about the size of the second, is bordered by enamel, 

 and possesses a short internal valley. 



The teeth of the second specimen, larger than those of the former one, consist of 

 the second to the fifth, or of the four intermediate ones, figure 15, and belong to the 

 permanent series. They are oblong-square curved columns, with cordiform triturating 

 surfaces, the outer border of which is devoid of enamel. A transverse fold or valley 

 extends from the inner side in all of them, and each presents a central islet, which is 

 crescentoid in the anterior pair of teeth, and minute and round in the posterior pair. 

 The specimen also retains parts of the first and sixth alveoli. 



The teeth of the third specimen, figure 16, are so much larger, and appear so differ- 

 ent from those of the other specimens, that they might be supposed to belong at least 

 to another species of the genus. They are teeth of a comparatively aged animal, and 

 consist of the second to the fifth inclusive of the series. The triturating surfaces are 

 transversely quadrate ellipsoidal, and bordered by enamel, except on the outer side. 

 All traces of the internal transverse fold or valley have been completely obliterated. 

 The remaining specimens, seven in number, consist of portions of lower jaws with 



