FROM THE FOREST BED OF EAST RUNTOJJ. 169 



leave no doubt that the teeth they supported were much larger and rounder than those 

 of the Beaver, even if this fact were not known from other specimens. 



All the upper cheek-teeth are preserved (figs. 2, 3), and, notwithstanding the chipping 

 and rounding of the enamel edges, their patterns may be distinguished (fig. 5). The 

 grinding-surface of each tooth is of a triangular form. Although the teeth have not 

 been much worn by use, it is only the single internal enamel fold of some of them 

 which remains open on the inner side ; the external enamel folds havino- all become 

 isolated, and it is only from other specimens that we know they were orio-inally open 

 to the exterior of the teeth. 



The anterior tooth (pm. 4), on each side, is distinctly larger than any of the others ; 

 it has one inner and three outer enamel folds. The second tooth (m. 1) has one inner 

 and two outer folds, the antero-external fold being very small. No doubt there was 

 originally a small third outer fold, as in the next tooth, which may have been worn 

 away during life, or has perhaps been broken away since it became fossilized. The 

 third tooth (m. 2) has one internal and three external folds, the anterior and posterior 

 external folds being very small. The fourth or hindermost tooth (m. 3) is about the 

 same width as the one next in front of it, but is longer from before backwards ; it has 

 one internal and four external folds. In the Beaver the anterior upper grinder (pm. 4) 

 is not so much larger than the second tooth (m. 1) as it is in the fossil, and the entire 

 series gradually decreases in size from before backwards ; while in this Trogontherium 

 the last tooth (m. 3) is larger than either of the two (m. 1, m. 2) next preceding it. 

 In the Beaver both the internal and external enamel folds of all the grinders are open 

 to the outer surface down to the bases of the teeth, so that they always appear as folds 

 on the grinding-surface, while in Trogontherium they very early become isolated. 



Summary of the Listinctive Characters of the Forest Bed Skull— The chief points of 

 difference between this skull and that of the Beaver may be thus summarized . 



The premaxillary region is wider and stronger. The frontals are longer, the parietals 

 are shorter, and the postorbital processes are further back. 



The anterior jugal buttress descends only about halfway down the maxilla, and not 

 to the alveolar margin as it does in the Beaver. 



The palatine foramina are much nearer to the front grinders than to the incisors, 

 and they are formed about equally by the maxilhie and premaxillee ; in the Beaver 

 they are midway between the incisors and grinders, and are almost wholly formed by 

 the premaxillse. 



The posterior palatine notch is further forwards than in the Beaver, and the pits in 

 the hinder extremities of the palatines are not seen in the Beaver. 



The under surface of the basioccipital has a median ridge with a fossa on each side 

 of it ; in the Beaver the basioccipital is entirely occupied by one large and deep fossa. 



The tympanic bullae are less inflated ; and the foramen magnum is more triangular 

 than in the Beaver. 



