ORAL CAVITY 23 1 



242, 244). The other hypothesis is that parts have been developed in the primi- 

 tive cone, giving, first, the triconodont shape. Next these three cones have been 

 shifted to the tritubercular position (fig. 241, A,B,C); and later other parts — 

 hypocone, lophs, etc. — have been added and these have been modified in different 

 directions. Each view has much in its favor. Embryology is not at all decisive, 

 while paleontology favors the latter view. In Dugong there is a fusion of enamel 

 organs for the molar teeth. 



Epidermal Teeth occur in cyclostomes and in larval amphibia and 

 in embryonic monotremes. In the cyclostomes they are cones of 

 cornified epithelium covering an underlying core of the integument; 

 they are differently arranged in the lampreys and myxinoids. In the 

 latter they are few, there being a single tooth on the 'palate' and two 

 chevron-shaped rows on the tongue. In the lampreys 

 nearly the whole inner surface of the oral hood is lined 

 with these teeth of varying shape, and there are a 

 varying number upon the tongue. These teeth are 

 used as a means of fastening the animals to their 



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Fig. 244. Fig. 245. 



Fig. 244. — Teeth of Chamydoselache (after R6se), showing a triconodont tooth 

 arising from the fusion of three simple teeth. 



Fig. 245. — Diagram of development of teeth in Omilhorhynhus, 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. 



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. 245), 

 but these are lost before birth. During their eruption the adjacent 

 epidermis becomes cornified, gradually extends beneath 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. 



