OF THE EARLIER VERTEBRATA. 41 



relation to certain parts of the skeleton within, which — • 

 greatly more developed in some of its contemporaries — led 

 to important 'results in the general structure of these ganoids, 

 and furnishes the true key to the character of the early ga- 

 noidal head. In such of the existing placoids as I have had 

 an opportunity of examining, the only portions of the dermal 

 skeleton of bone which conform in their arrangement to por- 

 tions of the interior skeleton of cartilage are the teeth, which 

 are always laid on a base of skin right over the jaws. There 

 is also an approximation to arrangement of a corresponding 

 kind, though a distant one, in those hook-armed tubercles of 

 certain species of rays which run along the vertebral column. 

 But in the shagreen by which the creatures are covered I have 

 been able to detect no such arrangement. Whether it occurs 

 on the fins, the body, or the head, or in the scale form, or 

 in that of the prickle, it manifests the same careless irregu- 

 larity. And on the head and body of the Gheiracanthus^ 

 and on all its fins save one, the shagreen-like scales, though 

 laid down more symmetrically in lines than true shagreen, 

 manifested an equal absence of arrangement in relation to 

 the framework within. On that one fin, however, — the cau- 

 dal, — the scales, passing from their ordinary rhomboidal to 

 a more rectangular form, ranged themselves in right lines 

 over the internal rays (fig. 9, a), and imparted to these such 

 strength as a splint of wood or whalebone fastened over a 

 fractured toe or finger imparts to the injured digit, — a pro- 

 vision which was probably rendered necessary in the case of 

 this important organ of motion, from the circumstance that 

 it was the only fin which the creature possessed that was not 

 strengthened and protected anteriorly by a strong spine. In 

 the Cheirolepis, — a contemporaiy fish, characterized, like its 

 congeners the Cheiracanthus and Diplacanthus, by shagreen- 

 like scales, but in which the spines were wanting, — we find 

 a farther development of the provision. In all the fins the 



