TOOTH-PLATES 



331 



The originally separate denticles develop as already explained 

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

 of dentine. Trabeculae of bony tissue ("trabecular dentine/' or 



^ A 



elf 



Fio. 164. — Illustrating the dental arrangements in young Lung-fishes. 

 (A and B after Semon, 1899.) 



A, roof ofmouth of a Ceratodus 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 ; C, Lepiclosiren, macerated upper jaw of young specimen, showing the pointed 

 cusps still present on the tooth-plates ; olf 1 , anterior naris ; oZ/2, posterior naris. 



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

 through the underlying mesenchyme, 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- 



