| OCR Text |
Show 708 DR. C I. FORSYTH MAJOR ON THE [June 1, viz. 4 and 6, 5 and 7, are opposed to each other in a similar way as in Hesperomyince; the oblique position of the rows is brought about by the different connections. The third, posterior, row, which remains essentially limited to the outer part of the posterior margin, shows on unworn teeth of both species, more distinctly in B. betsileoensis, its composition of two parts: an anterior somewhat stronger cusp, 5, and a posterior smaller one, 3; exactly as in the less reduced teeth of Hesperomyince,l. The lower molars agree with the upper ones, if we take into account that they are, of course, reversed. The anterior transverse row, restricted to the inner part, consists, as seen in unworn teeth of Brachyuromys betsileoensis (Plate X X X I X . fig. 7 b), of Winge's 1 and 4. The middle row consists internally of 2 and 5, which very soon become fused with the outer cusp 6. The posterior ridge is composed of 3 and 7. To sum up. The five transverse ridges of Trechornys, e. g.2, or of the Dipodide Zapus, are in Brachyuromys reduced to three, by the atrophy of 2 and 3 and their fusion with 4 and 5. Comparison of the Molars of the " Bhizomyes'"' (Tachyoryctes, Bhizomys) with those of Brachyuromys. It has been formerly stated that the molars of Brachyuromys ramlrohltra are almost identical with those of the African Tachyoryctes3. There is, however, a greater difference in vertical extension between the molars of the two genera than appears from my former statement: those of B. ramlrohltra are semi-hypselo-dont, whereas the very hypselodont molars of Tachyoryctes are on their way to become rootless. In Rhizomys the molars are less hypselodont than those of Tachyoryctes and somewhat more complicated than in both Tachyoryctes and Brachyuromys. This is particularly apparent in the molars of the upper jaw. While in the latter genus we have seen the upper molars to be composed essentially of three outer cusps (3 + 5 ; 2 + 4 ; 1), separated by two external folds, there are four cusps in Rhizomys and accordingly three folds, which later on become three enamel islets. In Rh. ba'dius the teeth, although as complicated in an unworn condition as those of the other species, become when worn more similar to tbe unworn teeth of Tachyoryctes and Brachyuromys, a fusion taking place in the anterior part of the crown. Moreover, the gradual disappearance of the islets from the crown proceeds at a quicker rate than in the other species of Rhizomys. In Tachyoryctes, m. 1 sup. of young specimens has a similar structure to that of all three molars of Rhizomys. But this complication too disappears very soon, so that somewhat worn anterior molars of 1 R. Hensel, " Beitrage z. Kennt. Saugeth. Sud-Brasiliens," Phys. Abb. k. Akad. d. Wiss. Berlin, 1873, pl. i. figs. 24 a, 25 a; pl. ii. I gs. 26 a, 28 a ; pl. iii. fig. 30 a.-Winge,' Gnavere fra Lagoa Santa, etc.,' pl. iii. figs. 1 a, 10 a, 12 a.- It is also apparent in several Muridas, e. g. Vromys, Conilurus (young), Chiruromys. 2 Winge, 'Gnavere fra Lagoa Santa,' p. 116. 3 P. Z. S. 1896, p. 979. |
