Fig. 212.—Section of the left auditory bulla and surrounding bones of a Bear (Ursus ferox). am, External auditory meatus; BO, basioccipital; Car, carotid canal; e, Eustachian canal; g, glenoid canal; Sq, squamosal; T, tympanic; t, tympanic ring. (From Flower, Proc. Zool. Soc. 1869.)
There is no caecum, a feature which marks off the Arctoidea from all Carnivora except the Viverrids Nandinia and Arctictis (occasionally). The brain is characterised by the possession of what Dr. Mivart has described as the "ursine lozenge," a tract about the middle of the hemispheres, defined posteriorly by the crucial sulcus, and formed by the emergence on to the surface of the brain of the hippocampal gyrus.
The Arctoidea are very widely distributed. But there are some curious exceptions. Thus there are no representatives of the group (as might be expected) in the Australian region; they are completely absent from Madagascar; while the true Bears (family Ursidae) are totally absent from Ethiopian Africa, and are only represented by a single species, Ursus ornatus, in the Neotropical region.
It is noteworthy that the Arctoidea never show spots or
cross stripes (save rings on the tail), which are so common a feature of the coloration of the Cat-like forms.
In bracketing together the three families which are described in the following pages, emphasis is laid upon a number of undoubtedly common features. Palaeontology seems, however, to suggest that the Mustelidae come nearer to the Viverridae. That the Bears and Dogs are connected by extinct annectent genera does not interfere with their present distinctness.
The systematic arrangement of these Carnivora is not easy. It may be useful, however, to give a method of arrangement for the convenient placing of the genera.
The most primitive group is perhaps that of the true Bears, family Ursidae; for in them the molars are two above and three below, and have thus not become diminished in number as in some of the other members of the order. Moreover, the Bears have lobate kidneys, which character, often occurring in the young of animals which when adult have smooth kidneys, may be looked upon as a primitive character. The feet furthermore are completely plantigrade. This family will contain only three genera, Ursus, Melursus, and Aeluropus.
Next comes the family Procyonidae, in several members of which one molar is lost below, though in others the more archaic formula is retained. The kidneys are simple. This family contains the American genera Procyon, Nasua, Bassariscus, Bassaricyon, Cercoleptes, and the Old-World form Aelurus.
The third family, Mustelidae, has the molar formula reduced to 1/2 or 1/1. The kidneys are simple except in the Otters. To this family are assigned the following genera:—Arctonyx, Conepatus, Meles, Mephitis, Taxidea, Mydaus, Mellivora, Helictis, Ictonyx, Mustela, Galictis, Grisonia, Putorius, Gulo, and the aquatic Lutra, Enhydris, and Aonyx.