A sub-group, distinguished from the above by the naked muffle, includes some very large and handsome species, which principally dwell in rocky mountain ranges, as _M. rufus_, the great Red Kangaroo, _M. antilopinus_, and _M. robustus_. The fourth premolar is of large or medium size in these forms. Remains of _M. giganteus_ occur fossil in the Pleistocene of Australia, where we also find the allied extinct _M. titan_, which attains somewhat larger dimensions. _M. robustus_ also dates from the same geological epoch, where it was accompanied by two allied types known as _M. altus_ and _M. cooperi_. The second group includes the larger Wallabies, which are smaller than the true Kangaroos, with a brighter and more variegated coloration. The palate is generally more incomplete than in the typical group; and in the molars the anterior talon is connected with the first transverse ridge by an external instead of a median longitudinal bridge. The members of this group are frequenters of forests and dense impenetrable brushes and scrubs, and hence are often called Brush Kangaroos, though a native name, “Wallaby,” is now generally applied to them. There are several species, of which _M. ruficollis_, _M. ualabatus_, _M. parryi_, and _M. agilis_ are the best known. _M. ualabatus_ and _M. parryi_ are found fossil in the Pleistocene deposits of Australia. In those beds we also meet with remains of several very large extinct species, which appear to be allied to those Wallabies in which the fourth premolar is large and elongated, all of them agreeing with the Wallabies in the absence of the median bridge between the first ridge and talon of the molars. These fossil forms comprise _M. brehus_, in which the skull was probably about one foot in length, and _M. rœchus_, and _M. anak_, which were of somewhat inferior dimensions. In the last-named species the length of the fourth upper premolar is equal to that of the first and half of the second molar.[81]