50 CERHBRAL DEVELOPMENT 



terniination of the cranial buckler, which, like the intermaxil- 

 laries, was thickly fringed with teeth, to form, as has been 

 already said, the creature's snout. 



The under jaws (10), — strongly-marked bones in at least 

 all the Dipterian and Celacanth genera, — we find represented 

 externally by massy plates, bearing, like those of the upper 

 jaw, their range of teeth. As shown in a well-preserved 

 specimen of the lower jaw of Holoptychius, in my possession, 

 they were boxes of bone enclosing a bulky nucleus of car- 

 tilage, which, in approaching towards the condyloid process, 

 where great strength was necessary, was thickly traversed by 

 osseous cancelli, and passed at the joint into true bone. It is 

 in the under jaws of the earlier ganoids that we first detect 

 a true union of the external with the internal skeleton, — of 

 the bony plates and teeth, which were mere plates and teeth of 

 the skin, with the osseous, granular walls, which enclosed at 

 ^east all the larger pieces of the cartilaginous framework of 

 tlie interior. The jaws of the rays and sharks, formed of 

 cartilage, and fenced round on their sides and edges by their 

 thin coverings of polygonal, bony points, are wholly internal 

 and skin-covered ; whereas the teeth, which rest on the soft 

 cuticular integument right over them, are as purely dermal 

 as the surrounding shagreen. Teeth and shagreen may, we 

 find, be alike stripped off with the skin. Now, in the ear- 

 lier ganoidal jaw, two sides of the osseous box which it com- 

 posed, — its outer and under sides, — were mere dermal plates, 

 representative of the skin of the placoids, or of their sha- 

 green ; while the other two, — its upper and inner sides, — 

 sf*fiixx to have been developments of the interior osseous wall;? 

 vmch covered the endoskeletal cartilage. Nor is it unworthy 

 <tf' notice, that the reptile fishes of the period had their ichthyic 

 teeth ranged along the edge of an exterior derrruil plate, which 

 covered the outer side of the jaw ; whereas their reptile teeth 

 were planted on a plate, apparently of interior development^ 



