VERTEBRATA. 77 



of the pachydermata ; but it is only in the extinct anoplotherium 

 among mammalia, that the three kinds of teeth are arranged 

 in an uninterrupted series, as in man. The superior incisors are 

 wanting in the ruminantia, and the inferior in the walrus. The 

 Ethiopian hog and certain bats lose their incisors at a particular 

 age. The canine teeth are absent in the rodentia, some ruminants, 

 and in most of the female solipeda. The rodentia have but two 

 incisors in each jaw, with the exception of the hare and rabbit, 

 which have them double in the upper jaw; the kangaroo has two 

 below and eight above ; the daman two above and four below. The 

 molar teeth are the most essential, and are the last to disappear: 

 hence the ornithorhynchus paradoxus, the tatu, and the two-horned 

 rhinoceros, are restricted to them. The molar teeth are renewed 

 eight times in the elephant, the incisors are shed twice in many 

 rodentia ; and most of the teeth are renewed once in the other 

 orders of the mammalia. In most mammalia which feed on animal 

 substances, the crowns of the teeth are entirely covered with enamel, 

 and only partially so in the phytophagous quadrupeds. 



The following is an outline of the process by which the teeth 

 are produced. About two months after conception, a gelatinous 

 substance lies alonsf each alveolar arch: at 'the third month this 

 substance is firmer, and lodged in a shallow groove in the bone. It 

 is next divided into separate pulps by transverse filaments passing 

 from one side of the alveolous to the other. These pulps are en- 

 closed in, and connected by vessels to a thin vascular membrane, 

 which, between the third and fourth month begins to secrete the 

 ossific laminae from its outer surface. This membrane, with its 

 contained pulp, is supplied from the dental vessels, and nerves, and 

 is surrounded by a thick vascular sac, separable into two layers ; 

 • he latter membrane is attached to the pulp only at its base, but is 

 firmly connected by its outer layer to the gum, from which it derives 

 its vascular and nervous supplies. It is from the inner surface of 

 the internal layer of this sac that the enamel is secreted, and at 

 this period it becomes thick and vascular, whilst the outer layer, 

 which is only rudimentary in man, secretes the crusta petrosa in 

 the graminivorous quadrupeds. After the enamel has been secreted, 

 both layers of this sac become wholly absorbed, hence they have 

 been termed the deciduous membranes, in contra-distincti< n to the 

 permanent, which are described as three in number, one being the 

 periosteum of the alveolus, another the periosteum of the root, 

 and the third the periosteum of the dental cavity, which secreted 

 the tooth. But in fact these three permanent membranes are simply 

 a continuation of the periosteum of the jaw which first lines the 

 alveolus, then descends, to form the periosteum of the root, and 

 lastly passes up in the form of a hollow cone to enclose the pulp. 

 The period at which the teeth appear in the human subject is very 

 variable, some children being born with two or more, whilst in 

 others they may not appear for two or even three years. From five 

 to eight months, however, is the most usual period ; they generally 



