OF THE EARLIER VERTEBRATA. 



51 



which covered its upper edge. It is further worthy of remark, 

 that while the teeth of the dermal plate, — themselves also der- 

 mal, seem as if they had grown out of it, and formed part of it, 

 — just as the teeth of the placoids grow out of the skin on 

 which they rest, — the reptile teeth within rested in shailow 

 pits, — the first faint indications of true sockets. 



That space included within the arch formed by the sweep 

 of the under jaws, which we find occupied in the osseous 

 fishes by the hyoid bones and the branchiostegous rays, was 

 filled up externally, in the Dipterians and Celacanths, and 

 in at least two genera of Cephalaspians, by dermal plates ; in 

 some genera, such as the DiphpteruSy by three plates ; in 

 others, such as the Iloloptychius and Glyptolepisy by two j 

 and in the Asterolepis, as we shall afterwards see, by but a 

 single plate. In the Osteolepis these plates were increased to 



Fig. 14. 



UNDER PAET OP HEAD OP OSTEOLEPIS.* 



five in number by the little plates 14, 14 (fig. 14), "whicii, 

 however, may have been also present in the Diplopterus, though 



* The jaws (10, 10), which exhibit in the print their greatest breadth, 

 would have presented in the animal, seen from beneath, their narrow 

 imder-edges, and have nearly faUen into the line of the sub-opercular 

 Dlates (13, 13). 



