TOOTH-PLATES 



331 



The originally separate denticles develop as already explained 

 (p. 324) in typical placoid fashion, giving rise to little hollow cones 

 if dentine. Tr.iU-rulae of Imny 1 issue (" trabecular dentine/' or 



^ 



olf 



FIG. 164. Illustrating the dental arrangements in young Lung-tislics. 

 (A audB after Semon, 1899.) 



A, roof of mouth of a GVratodns of stage 48, showing the separate conical teeth ; B, teeth of roof of 

 mouth from a slightly younger specimen (stage 46) after the soft tissues have been cleared away by the 

 action of dilute alkali ; ('. /.f /'/</<>.-)/> //. macerated upper .jaw of voting specimen, showing the i>ointd 

 cusps still piesent on (Jie tooth-plates ; o//l, anterior nai-is ; "//-. jiosterior naris. 



" pulp dentine ") spread inwards from the bases of these cones 

 through the underlying mesenchynie, so as to join up the various 

 denticles by a loose calcified spongework. As development goes on 

 the trabeculae of this thicken, the pulp-filled meshes become pro- 



