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. True cats of the genus _Felis_ were in existence before the close of the Miocene, and yet earlier related forms are known. Throughout the greater part of the Tertiary the remarkable type known as sabre-toothed cats were numerous and widely spread, and in South America they even lasted so far into the Pleistocene that it is probably true that they existed side by side with man. Some of them were as large as any existing cat and had upper canines six inches or more in length. Cats have no near relations upon the American continent, nor do they appear to have ever had many except the sabre-tooths. Of present species some fifty are known, inhabiting all of the greater geographical areas except Australia. They are tropical and heat loving, but the short-tailed lynxes are northern, while both the tiger and leopard in Asia, and puma in America, range into sub-arctic temperatures, and it is a curious anomaly that while Siberian tigers have gained the protection of a long, warm coat of hair, pumas from British America differ very little in this respect from those of warm regions.