272 HISTORY OF THE HUMAN BODY 



present-day selachians, and have been modified in form through 

 the change of function due to their position in and about the 

 mouth cavity. Teeth and placoid scales correspond closely in 

 structure and development, and consist of a basis of dentine, 

 or " ivory/' as it is often called, overlaid by enamel, the first 

 being formed from the corium, the latter from the epidermis. 



r In their original distribution as seen in fishes and amphibi- 

 ans they occur not only along the edges of the jaw, but also 

 in patches over the roof and floor of the mouth cavity, co- 

 extensive with the stomatodseum ; but in most reptiles and in 



mammals they are confined to a single row in each jaw. Cor- 

 responding to their origin the most primitive arrangement is 

 that of an imbricated pattern as in other scales, a condition 

 shown in many of the areas within the mouth cavity, and in 

 the jaw teeth of many selachians, where they appear in several 

 rows. Primarily, at the stage in which the jaws and skull 

 are cartilaginous, as in modern selachians, the teeth are 

 separated from one another, but in a slightly higher stage the 

 bases fuse for mutual support, thus forming a flat plate of 

 bone upon which the separate tooth elements appear as pro- 

 jecting points or cusps. This proceeding is repeated ontoge- 

 netically in a few cases, as in the paired vomers of the frog; 

 although usually by a shortening of the development the stage 

 at which the scales are separate is dropped out and the bony 

 plate develops as a single structure, so that in cases where the 

 cusps have become lost there is no indication of their dental 

 origin. Thus are formed the Hat bones that line the mouth 

 cavity, such as the vomers, palatines, pterygoids, and parabasal, 

 which in lower forms often retain their dentigerous character; 

 such is also the origin of the premaxillaries and maxillaries 

 that form the upper jaws, as well as that of the splint-like 

 dentare, and perhaps the angulare, that enclose Meckel's carti- 

 lage and form the mandible. (See Chap. V.) The cusps are 

 thus originally an integral part of the bony splints which bear 

 them, and in fishes, amphibians, and most reptiles the two 

 remain continuous, but in crocodiles and mammals, as well 

 as in the fossil toothed birds, the two become detached, and the 



