The third of the main groups in ascending order is the _family_, which ordinarily consists of a number of genera united by the possession of certain common characters, which, at the same time, distinguish them from other genera, though a single isolated genus may require a separate family for its reception. Just as it is often convenient to divide a genus into subgenera, so families containing many genera are usually divisible into _subfamilies_, as indicative of closer relationships within the family. The name of the family is formed from that of the genus first described or best known, with the termination _-idæ_, while that for the subfamily is _-inæ_. To take an example, all the genera of cats, living and extinct, are assembled in the family Felidæ (from the genus _Felis_) which falls naturally into two subfamilies. One of these, the Felinæ, includes the true cats, a very homogeneous group, both the existing and the extinct genera; the other subfamily, that of the highly interesting series of the “Sabre-tooth Tigers,” called the †Machairodontinæ, comprises only extinct forms. The fourth principal rank or grade is the _order_, distinguished by some fundamental peculiarity of structure and usually including a large number of families. Some of the orders, however, contain but a single family, a single genus, or even, it may be, a single species, because that species is in important structural characters so unlike any other that it cannot properly be put into the same order with anything else. Such isolation invariably implies that the species or genus in question is the sole survivor of what was once an extensive series. As in the case of the family and the genus, it is often necessary to recognize the degrees of closer and more remote affinity by the use of _suborders_. Existing Artiodactyla, or even-toed hoofed animals, an enormous assemblage, may conveniently be divided into four suborders: (1) Suina, swine and the Hippopotamus; (2) Tylopoda, the Camel and Llama; (3) Tragulina, “mouse-deer,” or chevrotains; (4) Pecora, or true ruminants, deer, giraffes, antelopes, sheep, goats, oxen, etc. In nearly all of the orders such subordinal divisions are desirable and it is frequently useful to employ still further subdivisions, like _superfamilies_, which are groups of allied families within the suborder, _sections_ and the like. In the Linnæan scheme, the next group in ascending rank is the _class_, which includes all mammals whatsoever, but the advance of knowledge has made it necessary to interpolate several intermediate grades between the class and the order, which, in the descending scale, are _subclass_, _infraclass_, _cohort_, _superorder_ and others, while above the class comes the _subkingdom_ of Vertebrata, or animals with internal skeletons, which includes mammals, birds, reptiles, amphibians and fishes.