the metacone; the corresponding cusps of the lower tooth being named protoconid, paraconid, and metaconid—the protoconid being here on the outer side of the crown. It is thus apparent that in the first, or haplodont type, as well as in the triconodont type, the upper and lower molars are alike; while in the simple tritubercular type they have a similar pattern, but with the arrangement of the cusps reversed. This simple tritubercular type occurs in the Mesozoic genus _Spalacotherium_ (Fig. 4, ₆ and ₇), and apparently in the existing _Chrysochloris_; but in the majority of tritubercular forms, while this primitive triangle forms the main portion of the crown, other secondary cusps are added, the homologies of which in the upper and lower teeth are somewhat doubtful. At the same time that we have the addition of these secondary cusps we also find trituberculism differentiating into a secodont and a bunodont series, according as to whether the dentition becomes of a cutting or a crushing type. Thus in the lower molars (Fig. 4, ₈ and ₉) we very frequently find the three cusps of the primitive triangle elevated and connected by cross crests, while there is an additional low posterior heel or talon, which may be termed the hypoconid. This tubercular-sectorial sub-type, as it is termed, is found in the lower molars of many Polyprotodont Marsupials and Insectivores, and it also occurs in the lower carnassial teeth of the true Carnivora. The presence of two cusps (inner and outer) to the talon converts this modification into a quinquetubercular form; while, by the suppression of one of the three primitive cusps, it develops into the quadritubercular type of the bunodont series. [Illustration: FIG. 5.—Diagram of two upper and two lower left quadritubercular molars in mutual apposition. The cusps and ridges of the upper molars in double lines, and those of the lower in black lines. The lower molars are looked at from below, as if transparent. _pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_, metacone; _ml_, protoconule; _pl_, metaconule; _prd_, protoconid; _hyd_, hypoconid; _pad_, paraconid; _med_, metaconid; _end_, entoconid. (After Osborn.)] In the upper molars the primitive triangle in the secodont series may remain purely tricuspid; but the addition of intermediate cusps, both in the secodont and bunodont series, may give rise to a quinquetubercular type; these intermediate cusps being respectively designated as the protoconule and metaconule (Fig. 5, _ml_, _pl_). Finally, in the bunodont series, the addition of a postero-internal cusp (Fig. 5, _hy_), termed the hypocone, forms the sextubercular molar. The following table exhibits, in a collective form, the names and relations of all the above-mentioned cusps, and the letters by which they are indicated in the figures:— UPPER MOLARS.