. A history of British fossil mammals, and birds . ar is as great as from this part to the articular con- 186 dyle; whilst, in the existing European Beaver, the dia-stema between the summit of the incisor and the first molaris little more than one-third of the extent from the incisorto the articular condyle. The lower incisor of the Trogon-therium not only differs in both absolute and relative size,but also in shape. The anterior, or outer enamelled part ofthe tooth, is more convex, and, in a transverse section ofthe tooth (fig. 73, a), describes a semicircle. The inner, ormesial s
. A history of British fossil mammals, and birds . ar is as great as from this part to the articular con- 186 dyle; whilst, in the existing European Beaver, the dia-stema between the summit of the incisor and the first molaris little more than one-third of the extent from the incisorto the articular condyle. The lower incisor of the Trogon-therium not only differs in both absolute and relative size,but also in shape. The anterior, or outer enamelled part ofthe tooth, is more convex, and, in a transverse section ofthe tooth (fig. 73, a), describes a semicircle. The inner, ormesial surface of the incisor, which in the existing andfossil Beavers is flat, is concave in the Trogontherium, asis also the outer surface of the tooth. A well-marked sub-generic distinction, viz. the dispro-portionate size of the anterior molar, is well shewn in thefigure above-cited, but I shall more particularly notice it inconnection with the instructive portion of the lower jaw ofthe Trogontherium in the collection of Charles Lyell, Esq., Fig. Trogontherium Cuvieri, nat. size. Fresh-water beds, Cromer. by whose kind permission the specimen is here figured anddescribed (figs. 72 and 73). It was discovered by Mr. Lyellin the blue clay which rests upon the Norwich crag at TROGONTHERIUM (JUVIERl. 187 Cromer. The stratum of clay Is eight feet thick, and con-tains pyrites: its upper part is at about high water-mark,and it forms the beach. Here, or in situ in the bhie clay,were discovered bones of the Mannnoth, Rhinoceros, Ox,Horse, and Deer. The Trogontherian relic is a portion of the right ramus ofthe lower jaw, containing half the root of the great in-cisor, and the three anterior molar teeth in situ. Theproportions of these teeth differ conspicuously from thosein the Beavers, both European and American. The an-tero-posterior and transverse diameters of the first grinderexceed by one-third those of the second grinder: boththe second and third molars are smaller in proportion to
Size: 2042px × 1223px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookaut, bookcentury1800, bookdecade1840, booksubjectpaleontology
