Fig. 34.—Skull of Dasyurus (lateral view). al.sph, Alisphenoid; ang, angular process of mandible; fr, frontal; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; oc.cond, occipital condyle; par, parietal; par.oc, paroccipital process; p.max, premaxilla; s.oc, supraoccipital; sq, squamosal; sq′, zygomatic process of squamosal. (From Parker and Haswell's Zoology.)
Fig. 35.—Upper and lower teeth of one side of the mouth of a Dolphin (Lagenorhynchus), illustrating the homodont type of dentition in a mammal. (After Flower and Lydekker.)
The teeth of the Mammalia are almost without exception "heterodont," i.e. they show differences of structure in different parts of the mouth. As a general rule, teeth can be grouped into cutting incisors, sharp conical canines, and molars, with a surface which is in the majority of cases suited for grinding. In this they contrast with the majority of the lower vertebrates, where the teeth are "homodont" (or, better, homoeodont), i.e. all more or less similar and not fitted by change of form to perform different duties. But there are exceptions on both sides. In
the Toothed Whales the teeth are homodont, as they are in the frog and in most reptiles; on the other hand, some of the remarkable reptiles belonging to Professor Huxley's order of the Anomodontia have distinct canines, and show other differentiations in their teeth.
A second characteristic of the mammalian dentition is the limited number of the teeth, which rarely exceeds fifty-four. Here again the Toothed Whales are an exception, the number of their teeth being as great as in many reptiles. In the Mammalia the number of the teeth is fixed (excepting of course for abnormalities), while in reptiles there is frequently no precise normal. Two regions may be distinguished in every tooth—the crown and the root; the latter, as its name denotes, is imbedded in the gum, while the crown is the freely-projecting summit of the tooth. The varying proportions of these two regions of the tooth enables us to divide teeth into two series—the brachyodont and the hypselodont; in the latter the crown is developed at the expense of the root, which is small; the hypselodont tooth is one that grows from a persistent pulp or, at any rate, one that is long open. Brachyodont teeth on the contrary have narrow canals running into the dentine. The primitive form of the tooth seems undoubtedly to be a conical single-rooted tooth, such as is now preserved in the Toothed Whales and in the canine teeth of nearly all animals. The development of the teeth, that is, the simple bell-shaped form of the enamel organ, seems to go some way towards proving this; but it is quite another question whether we can fairly regard the Whales as having retained this early form of tooth. In their case the simplification, as is so often the case where organs are simplified, seems to be rather degeneration than retention of primitive characters. But this is a matter which must be deferred for the present.
The incisor teeth are generally of simple structure and nearly always single rooted. In the Rodents, in the extinct Tillodontia and in Diprotodont Marsupials, they have grown large, and, as has been already stated, they increase in size continuously from the growing pulp. These teeth have a layer of enamel only on the anterior face, which keeps a sharp chisel-like edge upon them by reason of the fact that the harder enamel is worn away more slowly than the comparatively soft dentine. The
"horn" of the Narwhal is another modification of an incisor, as are the tusks of Elephants. Among the Lemurs the incisors are denticulate, and serve to clean the fur in a comb-like fashion. This is markedly the case in Galeopithecus. The incisors are sometimes totally absent, as in the Sloths, sometimes partially absent, as in many Artiodactyles, where the lower incisors bite against a callous pad in the upper jaw, in which no trace of incisors has been found.
Canine teeth are present in the majority of mammals, but are absent without a single exception from the jaws of the Rodentia. The canine tooth of the upper jaw is that tooth which comes immediately after the suture dividing the premaxillary from the maxillary bone. The canines are as a rule simple conical teeth, with but a single root; indeed they resemble what we may presume to have been the first kind of tooth developed in mammals. In this they resemble also as a general rule the foregoing incisors. But instances are known where the canines are implanted by two roots. This is to be seen in Triconodon, in the pig Hyotherium, in the Mole and some other Insectivores, and in Galeopithecus, where the incisors also may be thus implanted in the jaw. Furthermore, the simple condition of the crown of the tooth may be departed from. This is the case with a Fruit Bat belonging to the genus Pteralopex. In the more primitive Mammalia it is common to find no great difference between the canines and incisors; such is the case with the early Ungulate types of Eocene times, such as Xiphodon. In modern mammals, however, especially among the Carnivora, the canines tend to become larger and stronger than the incisors, and in some of the Cats and in the Walrus these teeth are represented by enormous offensive tusks. It is not rare for the canines of male animals to be larger than those of their mates. There are also cases such as the Musk-deer and the Kanchil where the male alone possesses these teeth, but only in the upper jaw. The teeth which follow the canines are known as the grinders or cheek teeth, or more technically as premolars and molars. These two latter terms separate teeth which arise at different periods, and their use will be explained later. In the meantime it may be pointed out that the cheek teeth are the teeth which show the greatest amount of variation in their structure; this is shown by the number and variety of the cusps in which
the biting surface ends. The grinding teeth vary from simple one-cusped teeth, precisely like canines, to teeth with an enormous number of separate tubercles. In the former case it is hard to distinguish between incisors, canines, and cheek teeth in the lower jaw, where no suture separates the bone. Moreover it is quite common for the first cheek tooth in the lower jaw to have the characters of a canine, while the true canine approximates in its form to the antecedent incisors. This is so, for instance, with the Lemurs, where the first premolar is caniniform, and the canine shares in the curious procumbent attitude which distinguishes the lower incisors of many of those animals.