DENTITION. 505 



is much elongated vertically, the neck of the tooth being deeply 

 imbedded in the socket. Such long crowned teeth are called 

 hypsodont in contradistinction to the first described teeth with 

 short crowns which are called brachyodont. It is evident that 

 to render a hypsodont tooth effective, it must gradually rise in 

 its socket as the crown is worn down. This is sometimes partly 

 effected by continued growth at the base of the tooth, the pulp 

 cavity remaining open and the pulp active. But hypsodont 

 grinding teeth are rarely rootless throughout life. They are 

 often rootless in the young animal, continuing to grow for a time, 

 but as a rule the roots are soon formed and growth ceases. In 

 such cases the subsequent elevation of the tooth to meet the 

 continual wear of the crown is effected in a manner which is not 

 fully understood. 



Hypsodont teeth are found constantly in some groups of the 

 Ungulata. In the Rodentia in which they also often occur, con- 

 siderable variety is found, closely allied species differing in this 

 respect. 



In hypsodont teeth the cusps and ridges are of course much 

 elongated vertically, and there are deep chasms and valleys be- 

 tween them. These are usually filled with cement (Fig. 264, V), 

 which confers strength and holds the elongated tubercles to- 

 gether. The result of this is that the surface of the worn tooth 

 presents a pattern caused by the three materials of different 

 degrees of hardness which compose the crown. These patterns 

 consist of the laminae of hardest material, enamel, bounding 

 areas of the next hardest substance, the dentine, and surrounded 

 by the softer cement which pervades the whole crown between 

 the cusp and ridges. 



Development of teeth (Fig. 266). There is formed in the 

 embryo at a comparatively early stage of development an in- 

 growth, in the form of a double lamina, of the lower layer (stra- 

 tum Malpighi) of the buccal epithelium. This ingrowth extends 

 along the whole length of the jaws and constitutes the primitive 

 tooth-band. There is frequently a groove in the lining of the 

 mouth placed immediately over it and called the dental groove 

 (zf) ; this, however, appears to be without significance in the 

 tooth development. 



The tooth-band has at first the appearance of a tubular gland 

 projecting into the subjacent mesoblast. The definitive tooth- 



