I246 CLASS MAMMALIA. 



observe that imbricated horny scales occur in the epidermis only in 

 the family Manidce among the Edentata; and flat horny shields, 

 with their edges in apposition, in the tails of the Beaver, Rats, and 

 certain Insectivores and Marsupials. The Armadillos and Glypto- 

 donts develop, however, a series of bony scutes articulating with 

 one another in the true dermis, which are covered by horny epi- 

 dermal shields ; the whole structure being thus precisely comparable 

 to that obtaining in the Crocodilia. Smaller separate bony scutes 

 also occur in the dermis of Mylodon. The horns of the Ruminants 

 and Rhinoceroses are entirely epidermal structures ; the former 

 being hollow sheaths enveloping bony cores, while the latter are 

 solid throughout. 



The dental system, as being of extreme importance for the deter- 

 mination of the extinct forms, must be noticed somewhat more fully. 

 Calcified teeth are developed in the great majority of Mammals, but 

 in the true Whales they occur only in the embryo, in Ornitho?'hynchus 

 they disappear in the adult, while in Echidna, Mam's, and Myrme- 

 cophaga, no traces of them have as yet been detected. In the adult 

 of Ornithorhynchus and Rhytina the function of teeth is discharged 

 by horny plates, or cornules on the palate. In all other forms, how- 

 ever, true teeth, which are developed only in the premaxilla, maxilla, 

 and dentary bones, are present ; and are usually composed of the 

 three elements, dentine, enamel, and cement, although occasionally, 

 as in the existing Edentates, the enamel is absent. The dentine, 

 or ivory, forms the chief constituent of most teeth. This is coated, 

 either completely or partially, in the majority of cases by a thin in- 

 vesting layer of the hard flint-like enamel, which is readily distin- 

 guished from the dentine by its bluish-white and translucent appear- 

 ance, while the outermost coat of cement, when present, is of a dull 

 opaque white, or buff, colour. The cement is frequently found only 

 as a thin coating at the roots of the teeth ; but it is very largely 

 developed in the crowns of the hinder teeth of many Ungulates. 

 In the teeth of the great majority of Mammals (as in fig. 11 26) the 

 crown, or exposed portion, is sharply defined by a construction 

 known as the neck from the root, or embedded portion ; but in those 

 teeth to be immediately noticed, which grow continuously, there is 

 no such distinction between the crown and the root. In no Mam- 

 mals are the teeth anchylosed to the bones of the jaw ; * and they 

 are invariably implanted in distinct alveoli, or sockets, which are, 

 however, very imperfect in certain Cetacea. In all young animals, 

 while the teeth are still growing, the inferior extremity of the root, 

 or roots, is widely open ; but in the majority of instances this aper- 

 ture becomes completely closed in the adult (fig. n 26). In certain 



1 Except, perhaps, the incisors of the Shrews. 



