The foot, or pes, consists normally of five toes, connected with the tarsus by means of five metatarsal bones, which closely resemble the metacarpals. In the Ruminants there are two principal metatarsals, and these are anchylosed in the adult, and carry two toes. In the Horse there is only one complete metatarsal supporting a single toe. As a rule, the number of digits in the hind-limb or foot is the same as that in the fore-limb or hand; but this is not always the case. In the Lions, Tigers, Cats, and Dogs, the posterior limb carries only four toes, the innermost toe or hallux being wanting. In the Quad-rumana, again, all the five toes are generally present, but the four outer toes are much longer than in Man, and the hallux is shorter than the other toes, and often opposable to them, so that the foot forms a kind of posterior hand. The hallux is also not uncommonly opposable in other cases.
The cranial bones are invariably connected with one another by sutures, and in no other examples than the Monotremes are these sutures obliterated in the adult. The differences of opinion which are entertained as to the fundamental structure of the skull are so enormous, that it will be best not to attempt here any detailed description of the skull of the Mammalia, more especially as there is as yet no universal agreement even as to the nomenclature to be employed. It is sufficient to remember that the skull is composed of a series of bony segments, which are often regarded as modified vertebrae. The occipital bone carries two condyles for articulation with the first cervical vertebra. The lower jaw is composed of two halves or rami, which are distinct from one another in the embryo, and may or may not be anchylosed together in the adult. However this may be, in no Mammal is the ramus of the lower jaw composed of several pieces, as it is in Birds and Reptiles, nor does it articulate with the skull by the intervention of an os quadratum. On the other hand, each ramus of the lower jaw in the Mammals is composed of only a single piece, and articulates with the squamosal element of the skull, or, in other words, with the squamous portion of the temporal bone.
Teeth are present in the great majority of Mammals; but they are only present in the embryo of the whalebone Whales, and are entirely absent in the genera Echidna, Mam's, and Myrmecophaga. In the Duck-mole (Ornithorhynchus) the so-called teeth are horny, and the same was the case in the extinct Rhytina amongst the Sirenia. In all other Mammals the teeth have their ordinary structure of dentine, enamel, and crusta petrosa, these elements being variously disposed in different cases, the enamel being occasionally wanting. In no Mammals are the teeth ever anchylosed with the jaw; and in all, the teeth are implanted into distinct sockets or alveoli, which, however, are very imperfect in some of the Cetacea.
Many Mammals have only a single set of teeth throughout life, and these are termed by Owen " monophyodont." In most cases, however, the first set of teeth - called the "milk" or "deciduous" teeth - is replaced in the course of growth by a second set of "permanent" teeth. The deciduous and permanent sets of teeth do not necessarily correspond to one another; but no Mammal has ever more than these two sets. The Mammals with two sets of teeth are called by Owen "diphyodont."
In Man and many other Mammals the teeth are divisible into four distinct groups, which differ from one another in position, appearance, and function; and which are known respectively as the incisors, canines, praemolars, and molars (fig. 356). "Those teeth which are implanted in the prae-maxillary bones, and in the corresponding part of the lower jaw, are called 'incisors,' whatever be their shape or size.
The tooth in the maxillary bone which is situated at or near to the suture with the praemaxillary, is the 'canine,' as is also that tooth in the lower jaw which, in opposing it, passes in front of its crown when the mouth is closed. The other teeth of the first set are the 'deciduous molars;' the teeth which displace and succeed them vertically are the 'praemolars;' the more posterior teeth, which are not displaced by vertical successors, are the 'molars' properly so called" (Owen). The deciduous dentition, therefore, of a diphyodont Mammal consists of only three kinds of teeth - incisors, canines, and molars. The incisor and canine teeth of the deciduous set are replaced by the teeth which bear the same names in the permanent set. The deciduous "molars," however, are replaced by the permanent "praemolars," and the "molars" of the permanent set of teeth are not represented in the deciduous series, only existing once, and not being replaced by successors. It has, however, been shown that in some diphyodont Mammals there may be certain of the anterior maxillary teeth in the permanent dentition, which are not represented by any predecessors in the deciduous series. This is the case, for example, with the first praemolar of the Dog.
Fig. 356. - Teeth of the right side of the lower jaw of the Chimpanzee (after Owen). i Incisors; c Canine tooth; pm Praemolars; m Molars.
All these four kinds of teeth are not necessarily present in all Mammals, and, as will be afterwards seen, the characters of the teeth are amongst the most important of the distinctions by which the Mammalian orders are separated from one another. The variations which exist in the number of teeth in different Mammals are usually expressed by a "dental formula," which presents the "dentition" of both jaws in a condensed and easily recognised form.