Of this proposition the Aye-Aye (_Chiromys_) among Lemurs, the Walrus among Seals, and the Narwhal among Dolphins form striking examples; since in all these forms the superficial characters of their dentition would entirely separate them from the animals with which all other evidence (even including the mode of development of their teeth) proves their close affinity. [Illustration: FIG. 4.—Molar teeth of Mesozoic Mammals (enlarged). Triconodont type—1, _Dromatherium_; 2, _Microconodon_; 3, _Amphilestes_; 4, _Phascolotherium_; 5, _Triconodon_. Tritubercular type—6, 7, _Spalacotherium_; 10, _Asthenodon_. Tubercular sectorial type—8, _Amphitherium_; 9, _Peramus_; 11-13, _Amblotherium_; 14 (?) _Amblotherium_. _pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_, metacone, in the upper teeth; and protoconid, hypoconid, paraconid, and metaconid in the lower. 6 and 15 are upper molars, and the rest lower molars. (After Osborn.)] _Trituberculism._—Recent researches, and more especially those of Professors Cope and Osborn, tend to show that almost all of the extremely different forms of tooth-structure found among Mammals may be traced to one common type, in which the crown of each tooth carried three cusps, and hence termed the _tritubercular_ type; these three cusps being arranged in a triangle, with the apex directed inwardly in the upper teeth (Fig. 4, ₆), and outwardly in the lower ones (Fig. 4, ₇). It is further probable that this tritubercular type was itself derived from a type of dentition in which the teeth were in the form of almost a quite simple cone; such a presumably primitive type of dentition—being apparently retained among some existing Edentates, like the Armadillos, while it is possible that we should regard the dentition of the existing Cetacea (Fig. 2) as a reversion to the same primitive type. None of the Mesozoic mammals at present known exhibit this simple conical type of teeth, although we have an approximation to it in the extremely generalised genus _Dromatherium_. Starting then from this presumed simple cone it appears that the teeth of _Dromatherium_ (Fig. 4, ₁) present the first stage towards trituberculism, the crown of each tooth having one main cone, with minute lateral cusps, and the root being grooved. In the next or true Triconodont stage (Fig. 4, ₃₋₅) the crown has become elongated antero-posteriorly, and consists of one central and two lateral cones or cusps, while the root is divided. From this the transition is easy to the tritubercular type, in which the three cusps, instead of being placed in a line, are arranged in a triangle; the upper teeth (Fig. 4, ₆) having one inner and two outer cusps, while the reverse condition obtains in those of the lower jaw (Fig. 4, ₇). These three cusps of the simple tritubercular tooth are collectively designated as the primitive triangle; in the upper tooth the inner cusp is termed the protocone, the antero-external one the paracone, and the postero-external