The difficulty so constantly met with in attempting to state concisely the details of classification, is well shown in this order, for its subdivisions rest less upon a few well defined characters than upon complex associations of a number of lesser and more obscure ones, a recapitulation of which would be tedious beyond the endurance of all but practiced anatomists.  For the present purposes it must be enough to say that bears and dogs have forty-two teeth in the complete set, of which four on each side above and below are premolars, and two above, with three below, are molars, but these teeth in bears have flatter crowns and more rounded tubercles than those of dogs, and the sectorial teeth are much less blade-like, this style of tooth being better adapted to their omnivorous food habits.  Bears, furthermore, have five digits on each foot and are plantigrade, while dogs have but four toes behind and are digitigrade.  These differences are less marked in some of the smaller arctoids, which may have as few as thirty-two teeth, and come very near to dogs in the extent of the digital surface which rests upon the ground in walking.

In distinction from these, Aeluroidea never have more than two true molars below, and the cusps of their teeth are much more sharply edged, reaching in the sectorials the extreme of scissor-like specialization.  In all of them the claws are more or less retractile, and they walk on the ends of their fingers and toes.

Cats are distinguished from the remainder of this section by the shortness of the skull, and reduction of the teeth to thirty, there being but one true molar on each side, that of the upper jaw being so minute that it is probably getting ready to disappear.

Civets, genets, and ichneumons are small as compared with most cats; they are fairly well distinguished by skull and tooth characters; their claws are never fully retractile, and many have scent glands, as in the civets.  No member of this family is American.

Hyaenas have the same dental formula as cats, but their teeth are enormously strong and massive, in relation to their function of crushing bone.

No carnivore has teeth so admirably adapted to a diet of flesh as the cat, and, in fact, it may be doubted if among all mammals, it has a superior in structural fitness to its life habits in general.

The Felidae are an exceedingly uniform group, although they do present minor differences; thus, some species have the orbits completely encircled by bone, while in most of them these are more or less widely open behind; in some the first upper premolar is absent, and some have a round pupil, while in others it is elliptical or vertical, but if there is a key to the apparently promiscuous distribution of these variations, it has not yet been found, and no satisfactory sub-division of the genus has been made, beyond setting aside the hunting-leopard or cheetah as Cynaelurus, upon peculiarities of skull and teeth.