TEETH OF MONOPI1YODONTS. 271 



developed in the dog and other Carnivora, although they are 

 given, likewise, to many vegetable feeders for defence or combat ; 

 e. g., Musk-deer. Molar teeth, which are adapted for mastica- 

 tion, have either tuberculate, or ridged, or flat summits, and 

 usually are either surrounded by a ridge of enamel, or are tra- 

 versed by similar ridges arranged in various patterns. Certain 

 molars in the Dngong, the Mylodon, and the Zeuglodon, are so 

 deeply indented laterally by opposite longitudinal grooves, as to 

 appear, when abraded, to be composed of two cylindrical teeth 

 cemented together, and the transverse section of the crown is 

 bilobed. The teeth of the Glyptodon were fluted by two analogous 

 grooves on each side, fig. 214. The large molars of the Capybara 

 and Elephant have the crown cleft into a numerous series of com- 

 pressed transverse plates, cemented together side by side. The 

 modifications of the crown of the molar teeth are those that are 

 most intimately related to the kind of food of the animal possess- 

 ing them. Thus, in the purely carnivorous mammals, the prin- 

 cipal molars are simple, trenchant, and play upon each other like 

 scissor-blades. In the mixed feeding species, the working surface 

 of the molars becomes broader and tuberculated ; in the insectivo- 

 rous species it is bristled with sharp points ; and in the purely 

 herbivorous kinds, the flat grinding surface of the teeth is com- 

 plicated by folds and ridges of the enamel entering the substance 

 of the tooth, the most complex forms being presented by the 

 Elephants. 



§ 219. Teeth of Monophyodonts. A. Monotremata. — The sub- 

 stances serving for teeth in the Orni- 211 

 thorhynchus are of a horny texture, 

 consisting of close-set, vertical hollow 

 tubes, resembling the outer compact 

 tissue of baleen or ( whalebone.' They 

 are eight in number, four in the upper, 

 and as many in the under jaw. The 

 anterior tooth of the upper jaw is ex- 

 tended from behind forward, but is low, 

 very narrow, and four-sided. The 

 corresponding tooth in the lower jaw, 

 fig. 211, b, is rather narrower, and 

 retains longer its trenchant edge. At 

 a distance from the anterior tooth, equal 



to its own length, is situated the horny «**"*»■»« ***• ornithorhyncus. 

 molar, ib. c, which consists of a flattened plate of an oblong sub- 

 quadrate figure. The corresponding tooth in the lower jaw is 



