A very general feature of the teeth of the Mammalia is what is usually termed the diphyodont dentition. In the majority of cases there are two sets of teeth developed, of which the first lasts for a comparatively short time, and is termed on account of its usual time of appearance the "milk dentition"; this is replaced later by the permanent dentition. In lower vertebrates the teeth are replaced as worn away. There is not, however, so great an antithesis in this matter between the Mammalia {52} and other vertebrates as was at one time assumed. But in order to explain this very important part of the subject it will be necessary to give some account of the development of the teeth. The type selected is the Hedgehog, which has been recently and carefully described by Dr. Leche of Stockholm, which type has furthermore the advantage of being a "central" type of mammal. The first step in the formation of the teeth is a continuous invagination of the epithelium covering the jaw to form a deepish wall of tissue running in the thickness of the jaw; this is perfectly continuous from end to end of the lower jaw. From this "common enamel germ" (_Schmelzleiste_ of the Germans[26]) "special enamel germs" (_Schmelzorgane_, enamel organs) are developed here and there as thickenings in the form of buds {53} which arise on the outer side of the fold of epithelium and some way above its lower termination. These ultimately acquire a bell-like form, and are as it were moulded on to a thickened concentration of the dermis beneath; they then become separate from the downgrowth of the epithelium whence they have arisen. Finally, each of the eight germs becomes one of the milk teeth of the animal. The lower end of the sheet of invaginated epithelium, the common enamel germ, is the seat of the formation of the second set of teeth, of which, however, in the animal under consideration, there are only two in each jaw. But corresponding to each of the enamel germs of the milk dentition, with the exception of the first two molars, there is a slight thickening of the end of the common enamel germ, which at a certain stage is indistinguishable from the thickening which will become one of the permanent teeth. We have thus the diphyodont arrangement. But this does not exhaust the series of rudimentary teeth, though no more come to maturity than those whose development has already been touched upon. In the upper jaw a small outgrowth of the common enamel germ arises above and to the outer side of the enamel germ of the third milk incisor; this does not develop any further, but its resemblance to the commencing germ of a tooth seems to indicate that it is the remnant of a tooth series antecedent to the milk series. Furthermore, there are indications in the fourth premolar of a fourth series of teeth posterior in appearance to the permanent dentition. We arrive therefore at the important conclusion that although here as elsewhere there are only two sets of calcified teeth ever