TEETH 



13 r 



Originally, in vertebrates, the teeth were for seizing and holding 

 prey. Grinding and cutting teeth, tusks, and fangs, are all modifications 

 of the primitive mouth trap. 



The number of these holding teeth is indefinite in elasmobranchs, 

 which may have as many as one hundred. They are not attached to 

 the jaws, but merely imbedded in the skin of the mouth. They are all 

 about aUke; and when one is lost, another moves forward into its place. 



In teleosts, the number of teeth is somewhat reduced, although all 

 parts of the mouth and even the pharynx may carry them. The special 

 advance made by the teleosts is to set the teeth more firmly by fusing 

 their bases with the membrane bones of the mouth. 



Fig. 122. — Comparison in development and structure between a placoid scale and a 

 tooth, a, b, and c represent the scale; d, e, and / the tooth. In all the figures the 

 epidermis is dotted, but its stratmn germinativiitn is represented by a layer of large 

 cells with nuclei; and the cutis is presented as composed of fibers with scattered cells. 

 X, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. (From 

 Wilder's "History of the Human Body," Henry Holt & Co.) 



Amphibia still farther reduce the number of teeth, but retain them on 

 premaxilla, maxilla, mandible, vomer, and palatine bones, and more 

 rarely on the parasphenoid. But toads have no teeth whatever. A strik- 

 ing feature of certain ancient and long extinct amphibians, the labyrin- 

 thodonts, which arose in the Coal Period and survived into the Triassic, 

 was the enormously complicated folding of the tooth enamel and dentine, 

 which anticipated, yet went far beyond, the similar arrangement in some 

 mammals. 



Reptiles make two important advances toward the condition in 

 mammals. Some of them, like some of the amphibians, have their teeth 

 set on a ledge on the inner side of the jaw— pleurodont dentition. Or they 

 may have the tooth set directly on the bone, acrodont dentition. But the 

 crocodiles and some fossil reptiles attain to a thecodont dentition, in 



