Dental Formulae.—With few exceptions the right and left sides of the jaws have the same numbers of incisors, canines, pre molars and molars respectively. Consequently the "dental f or mula" as ordinarily written represents only one side of the whole dentition and the total number of teeth is equal to twice the sum of the numbers in the "formula." The adult dental formula of man, for example : On the other hand, a secondary increase in number of molars occurs in several groups. Thus in anthropoids and man a fourth molar is sometimes developed. The high number of simple teeth in the toothed cetaceans is the result of degenerative specialization.
Molars.—Primitive insectivorous molars are small and bear V-shaped cusps with sharp little blades; in shearing types the molars are reduced in number but are large, with one or two long blades; in omnivorous types either the cusps become rounded, often connected with other cusps by low ridges as in anthropoid apes and man, or they become conic and multiplied in number as in the wart-hog; or the whole crown becomes a swollen mass with low cusps as in the bear; in herbivorous types the molar crowns acquire crests and hillocks, arranged in patterns characteristic of the different species.
In 1883 E. D. Cope observed that the teeth and skeletons of the carnivores and primitive ungulates of the Lower Eocene were far less dissimilar than those of their modern descendants, and especially that, while the patterns of their upper molars were already diversified, there were three main cusps or elevations, two on the outer and one on the inner side of the upper molars, evi dently homologous in both carnivores and herbivores. This was the "tritubercular" upper molar type of the Eocene mammals. A fourth main cusp on the hinder inner side of the upper molars could be seen in earlier stages of evolution in the carnivores and in more advanced stages in the primitive herbivorous mammals.
In the lower jaws Cope observed that in both carnivores and herbivores each lower molar tended to conform to a type in which the tooth crown was divided into two moieties arranged in tandem : the first or anterior moiety (the trigonid) elevated and supporting a triangle of cusps, of which the chief lay on the outer side of the crown, the other two on the inner; the second or posterior moiety (the talonid) depressed below the trigonid, con sisting chiefly of a central concavity flanked on the inner and outer sides by single cusps. This central type of lower molar Cope called tuberculo-sectorial. Among recent mammals the tuberculo sectorial pattern may still be seen in the lower molar teeth of civets, opossums, hedgehogs and other flesh-eating and insectiv orous mammals.
Origin of the Mammalian Locomotor Apparatus.—The locomotor machinery of mammals, like that of other vertebrates, involves four closely interconnected systems : (I) the motor ele ments proper ; (2) the passive or supporting elements; (3) the combustion; (4) the activating, controlling and directive system.
The motor elements include the striped or red muscle cells and the parts built up from them, muscle fibres, muscles and muscle systems (see MUSCULAR SYSTEM). The supporting elements in clude: (a) the sheaths and connective tissue surrounding the active elements; (b) the tendons that attach the muscles to the bones; (c) the ligaments that tie the parts of the skeleton together and (d) the articulated bony skeleton, including the backbone, skull, ribs, sternum, pectoral and pelvic girdles and limb bones.
The locomotor apparatus of mammals, like that of lower ver tebrates, is typically adapted for quadrupedal progression by run ning on relatively open ground, though many forms have learned to progress in special ways : leaping, climbing, volplaning and even flying, swimming, digging, tunneling. But no matter how elabo rate the locomotor mechanism may be, it has all been evolved from the simple crawling mechanism of the earliest amphibians, and this in turn from the undulatory movements of certain air breathing, lobe-finned fishes of Devonian age.
When the earliest amphibians crawled up out of the water the fore and hind paddles were bent downwards to assist the wriggling movement of the body. At first, the limbs sprawled widely. In such forms the humerus, or first segment of the skeleton of the forelimb, had a very short shaft and widely-expanded ends. There was a sharp bend at the elbow and the radius, or front forearm bone, articulated on the under side of the humerus rather than on its further end. Similarly in the hind-limb, the femur was short and was held widely out from the body.