2l6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



and there are a varying number upon the tongue. These teeth are 

 used as a means of fastening the animals to their prey, and those of the 

 myxinoid tongue are used for boring into the fishes on which these 

 animals feed. 



In the larval anura (the larval Siren is said to resemble them) the 

 edges of the jaws are armed with cornified papillae, serving as teeth, the 

 arrangement of which varies in different genera. They are frequently 

 aggregated in dental plates, used in scraping the algae from submerged 

 objects. They are not related to the teeth of cyclostomes. 



In the embryo monotreme teeth are formed as in 

 other mammals, of a multituberculate type, with a 

 normal enamel organ (fig. 219), but these are lost 

 before birth. During their eruption the adjacent epi- 

 dermis becomes cornified, gradually extends beneath 



FIG. 218. FIG. 219. 



FIG. 218. Teeth of Chlamydoselache (after Rose) showing a triconodont tooth arising 

 from the fusion of three simple teeth. 



FIG. 219. Diagram of development of teeth in Ornithorhynchus, after Thomas and 

 Poulton. a, tooth covered with enamel organ, beneath oral epithelium; b, just before 

 eruption; c, tooth erupted; d, edges of epithelium cornified; e, horny plate formed, contains 

 the tooth;/, tooth lost, plate separated from its surroundings. 



each tooth and after the loss of the true tooth this forms a horny 

 plate, used, like those of many birds, in holding and crushing the 

 food. 



In this connection mention may be made of the baleen or 'whale- 

 bone' of the balenid whales. This takes the form of large plates of 

 horny material, attached in series to the margins of the upper jaw, so 

 that with their fringed ends and edges they serve as strainers to extract 

 the plankton (minute floating life) from the sea. This baleen is formed 

 by the agglutination of enormously developed cornified papillae. 



Egg Teeth. In the embryos of certain lizards and snakes one of the median 

 teeth of the first dentition of the premaxillary region projects from the mouth and 

 is used for the rupture of the egg shell, thus allowing the escape of the young. 

 In the turtles, Sphenodon, crocodiles, birds, and monotremes an egg tooth is formed 

 on the upper surface of the beak which is used for the same purpose. However, it 

 differs greatly as it is but a thickening, often calcified, of the epidermis (Fig. 195). 



