216 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 papilla, serving as teeth, the 
arrangement of which varies in different genera. They are frequently 
aggregated in dental plates, used in scraping the alge 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 
op) tr ch 
“py ee 
Fic. 218. Fic. 2109. 
Fic. 218.—Teeth of Chlamydoselache (after-Rése), showing a triconodont tooth arising 
from the fusion of three simple teeth. 
Fic. 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 papille. 
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). 
