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Show 716 DR. C. I. FORSYTH MAJOR ON THE [June 1, connects itself, as often in Hesperomyince, with the postero-internal cusp 5, with which it becomes fused at an early date (in another Malagasy genus, Gymnuromys, it remains independent). Nor is the antero-internal cusp, 1 + 4 , simple, although it is undivided in Brachyuromys. In the Hesperomyince it presents itself frequently as a simple cusp, but in several forms, " H. ratti-ceps," 1 " H. longlcaudatus " 2 " H. tumldus," 3 it appears as composed of two parts separated by an enamel islet-an anterior smaller cusp (Winge's 1, Osborn's paraconid), and a posterior larger cusp (the antero-internal cusp, 4). In Nesomys the original compound structure of this cusp (14-4) is likewise shown, in moderately worn molars, by an enamel islet; and in very young B. betsileoensis it is foreshadowed by a slight depression of the enamel. Prom this comparison of the lower molars of Nesomys with those of Brachyuromys betsileoensis and Hesperomyince, it results that the molars of tbe former approach closer to certain forms of Hesperomyince in the conformation of their pattern, the latter to others. The internal enamel-folds remain for a longer time open on the inner side in the teeth of Brachyuromys and several Hesperomyince than in Nesomys. In the relative positions of outer and inner cusps Brachyuromys differs from some other Hesperomyince. In both B. betsileoensis and B. ramirohilra the posterointernal cusp, 5, stands directly opposite the interspace (inlet) dividing the two outer cusps (6 and 7), so that it comes to alternate with the latter, as in " H. vulpinus," 4 H. squamipes"5; in these same Hesperomyince and in Brachyuromys 3 is more developed, whilst 2 is atrophied. In other Hesperomyince the principal outer and inner cusps are almost opposite each other, as in Nesomys. Brachyuromys agrees more with " II. vulpinus " 6 and " H. arenlcola ? " 7 (the former is somewhat hypselodont; the fig. of the latter presents a much worn molar); Nesomys with " H. ratticeps," 8 " H. longlcaudatus^ 9 " H. suhterraneus " 10. The form of " H. squamipes " u agrees with both of them. As was pointed out above, a still more advanced atrophy of 1, 2, and 3 leads us to such simple forms amongst Muridae as Cricetus, Mystrornys, "Hesperomys expulsus," la " Habrothrlx lasl-urus," 13 " Hesp. nasutus" ]4 in which the pattern of the crown shows essentially only four cusps. Winge is doubtful whether the presence or the absence of the " transverse ridge " between the outer cusps, 4, 5, in upper, and the inner cusps iu lower molars is the primitive condition in Muridae ; adding, " its presence in Sminthus, Scirtetes, and several other low Eodents leads to the assumption that this condition is the primitive one in Muridae; whereas the absence of this ' transverse ridge' in the lowest 1 Hensel, I. c. pl. i. fig. 2b b. 2 //;. pl. ii. fig. 28 b. 3 lb. pl. iii. fig. 30 b. 4 Hensel, I. c. pl. i. fig. 23 b. 3 lb. pl. i. fig. 24 b. 6 lb. pl. i. fig." 236. ' lb. pl. ii. fig. 27 b. 8 lb. pl. i. fig. 25 b. 9 lb. pl. ii. fig. 28 b. i° lb. pl. iii. fig. 31 b. " lb. pl. i. fig. 24 6. 12 Winge, 'Gnavere fra Lagoa Santa,' pl. ii. fig. 4 a . 13 lb. pl. ii. fig. 11 a. 11 Hensel, 'Beitr. Kenntn. d. Saugeth. Siid-Bras.' pl. iii. figs 19, 29. |