6 PROEESSOK OWEN ON THE CHAKACTEKS, ETC. 



periosteum to the organized cement which invests the fang or fangs 

 of the tooth. 



True teeth implanted in sockets are confined, in the Mammalian 

 class, to the maxillary, premaxillary, and mandibular or lower max- 

 illary bones, and form a single row in each. They may project 

 only from the premaxillary bones, as in the Narwhal ; or only from 

 the lower maxillary bone, as in Ziphius ; or be limited to the supe- 

 rior and inferior mamillaries and not present in the premaxillaries, 

 as in the true Euminantia and most Bruta (Sloths, Armadillos, 

 Orycteropes). In most Mammals teeth are situated in all the 

 bones above mentioned. 



The teeth of the Mammalia usually consist of hard unvascular 

 dentine, defended at the crown by an investment of enamel, and 

 everywhere surrounded by a coat of cement. 



The coronal cement is of extreme tenuity in Man, Quadrumana 

 and the terrestrial Carnivora ; it is thicker in the Herbivora, espe- 

 cially in the complex grinders of the Elephant. 



Vertical folds of enamel and cement penetrate the crown of the 

 tooth in the ruminating and many other Ungulata, and in most 

 Rodents, characterizing by their various forms the genera of those 

 orders. 



No Mammal has more than two sets of teeth. In some species 

 the tooth-matrix does not develope the germ of a second tooth, 

 destined to succeed one into which the matrix has been converted ; 

 such a tooth, therefore, when completed and worn down, is not 

 replaced. The Sperm Whales, Dolphins, and Porpoises are limited 

 to this simple provision of teeth. In the Armadillos and Sloths, 

 the want of generative power, as it may be called, in the matrix is 

 compensated by the persistence of the matrix, and by the uninter- 

 rupted growtli of the teeth. 



In most other Mammalia, the matrix of the first-developed tooth 

 gives origin to the germ of a second tooth, which sometimes dis- 

 places the first, sometimes takes its place by the side of the tooth 

 from which it has originated. 



All those teeth which r.re displaced by their progeny are called 

 ' temporary,' deciduous, or milk-teeth ; the mode and direction in 

 which they are displaced and succeeded, viz. from above downwards 

 in the upper, from below upwards in the lower, jaw, in both jaws 

 vertically— are the same as in the Crocodile ; but the process is 

 never repeated more than once in any mammalian animal. A con- 

 siderable proportion of the dental series is thus changed ; the 

 second or ' permanent ' teeth having a size and form as suitable 



