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Show 1899.] rLEXOUONT MOLARS OF MAMMALS. 559 with again in many Insectivora (Talpa, Tupaidae, Soricidae, ifec), and likewise in the Chiroptera, especially in Vespertilionidae, the most numerous and ancient family. In all these groups the molars differ from those of Proteodidelphys only by the greater or lesser development of cusp ma, by the suppression of cusp ai or its fusion with ae, and by the varying degree of simplification of the posterior lobe. Fig. 3. "HT^ *y*a Cyonasua arycntina: fifth right lower molar, superior (a) and external (b) aspect, nat. size.-Eocene ; Patagonia. Another branch, likewise originating from the most primitive Microbiotheridae, are the diprotodont Marsupials, which comprise the extinct Multituberculata of the Northern Hemisphere and Argentina, the numerous Paucituberculata of South America, and the Diprotodonts of Australia (Hypsiprymnoi'dea). Their most primitive type is that of the Garzonidae. The lower molars of Garzonia or Halmariphus (fig. 4) are not distinguished by any - ote syrL^L Halmariphus didelpkoid.es : fifth right lower molar, superior (a) and external (h) aspect, eight times nat. size.-Eocene ; Patagonia. essential character from those of the Didelphyidye ; their teeth exhibit the six cusps of those of Proteodidelphys, with an almost similar disposition and with the same external cingulum, c. Some species depart slightly from this form by the internal displacement of the two median cusps, the anterior and the posterior, so that each molar presents on the internal margin a range of four cusps, as can be seen in the molars of a Cretaceous species of Halmariphus, or a nearly related genus (fig. 5). In the Epanorthidae the paired cusps ae, ai, and pe, pi, are connected, forming two semicircular crests. In the Abderitidae the same cusps constitute two feebly accentuated, transverse crests. The slightly more recent Diprotodonts of the Parana deposits (Zygolestes) exhibit the same crests more accentuated ; they are still more developed in the existing South-American genus Ccenolestes of 0. Thomas, the molars of which have assumed the same form as those of the Australian |