But although this group contains the smallest number of forms, the varieties of the domestic dog are endless, and no part of the world is without a species of the genus, except certain islands, such as the West Indies, Madagascar, the Polynesian isles, New Zealand and the Malayan archipelago; in these territories there is no indigenous dog. I speak of dogs in its broad sense of Canis, including wolves and foxes.
The proper position of the Cynoidea should be between the bears and the cats, as in their dentition they approximate to the former, and in their digitigrade character to the latter; but, with a view to make this work concurrent with that of Jerdon's, I have accepted the position assigned by him, though it be a little out of place.
The general form of the skeleton of a dog resembles that of a feline, though the limbs may be to a certain extent longer; they also walk on the tips of their toes, but their claws are not retractile, although the ligament by which the process of retraction in the cat is effected is present in a rudimentary form, but is permanently overpowered by the greater flexor muscles. A dog's paw is therefore by no means such a wonderful piece of mechanism and example of power as that of the cat, but is feeble in comparison, and is never used as a weapon of offence, as in the case of felines, the prey being always seized by the teeth.
The skull partakes of the characteristics of both cat and bear. It departs from the simple cutting dentition of the former by the addition of two tuberculated molars in each upper jaw, or one more than the rudimentary molar in the cat, whilst the lower jaw has two extra molars on each side; the premolars are also in excess, being four in number on each side of the upper and lower jaws, whereas in the feline there are three above and two below.
There is also a difference in the lower carnassial or first molar, which impinges on the upper carnassial or fourth premolar; it has a protuberance behind, termed the heel, which is prominently marked, but it is in the molars in which the greatest deviation from the specially carnivorous dentition occurs. The incisors are somewhat larger than, but the canines and premolars approximate to, those of the felines; the crown of the incisors is cuspidate, and the premolars increase gradually in size, with the exception of the fourth in the upper jaw, the carnassial, which is treble the size of the one next to it.
But it is in the molars that we find the similarity to the semi-herbivorous bears. The last two molars on each side of the upper and lower jaws are true grinders, divided into four cusps, which suits the dog to a mixed diet.
Of course the increased number of teeth (the dog has forty-two against thirty of the cat) necessitates a prolonged muzzle, and therefore the skull has more of the bear than the cat shape. The nasal bones are long, the zygomatic arch smaller, but it has the ear-bulb or bulla tympani, so conspicuous in the cat and wanting in the bear, yet the character of the aperture of the ear or auditory meatus approaches that of the latter, as the margins of its outer aperture are somewhat prolonged into a short tube or spout, instead of being flush, as in the felines. Then the bony clamp or par-occipital process, which in the cats is fixed against the hinder end of the bulla, is in the dogs separated by a decided groove.
The intestinal peculiarities of this section consist of a very large cæcum or blind gut, which is small in the cats and wholly absent in the bears, and in the very long intestines. Some have a sub-caudal gland secreting a pungent whey-like matter.
Muzzle obtuse; tail short; no caudal gland.