4 THE VERTEBRATE SKELETON. 



In a few animals, such as Lampreys and Ornithorhynchus, 

 the jaws bear horny tooth-like structures of epidermal origin. 



The enamel of teeth and of placoid scales is also epi- 

 blastic in origin 1 , and it may be well at this point to give 

 some account of the structure of teeth, though they are 

 partly mesoblastic in origin. The simplest teeth are those 

 met with in sharks and dogfish, where they are merely the 

 slightly modified scales developed in the integument of the 

 mouth. They pass by quite insensible gradations into normal 

 placoid scales, such as cover the general surface of the body. 

 A placoid scale 2 is developed on a papilla of the dermis 

 which projects outwards and backwards, and is covered by the 

 columnar Malpighian layer of the epidermis. The outer layer 

 of the dermal papilla then gradually becomes converted into 

 dentine and bone, while enamel is developed on the inner side 

 of the Malpighian layer, forming a cap to the scale. The 

 Malpighian and horny layers of the epidermis get rubbed off 

 the enamel cap, so that it comes to project freely on the sur- 

 face of the body. 



As regards their attachment teeth may be (1) attached 

 to the fibrous integument of the mouth, or (2) fixed to the 

 jaws or other bones of the mouth, or (3) planted in grooves, 

 or (4) in definite sockets in the jaw-bones (see p. 107). 



Teeth in general consist of three tissues, enamel, dentine 

 and cement, enclosing a central pulp-cavity containing blood- 

 vessels and nerves. Enamel is, however, often absent, as in all 

 living Edentates. 



Enamel generally forms the outermost layer of the crown 

 or visible part of the tooth ; it is the hardest tissue occurring 

 in the animal body and consists of prismatic fibres arranged at 

 right angles to the surface of the tooth. It is characterised by 

 its bluish- white translucent appearance. 



1 The enamel of the pharyngeal teeth of some Teleosteans is hypo- 

 blastic in origin. 



2 See also p. 71. 



