42 CEREBRAL DEVELOPMENT 



riclily-enamelled dermal covering was arranged in lines over 

 the rays (fig. 9, 6) ; and the scale, which 

 assumes in the fins, like the scales on the 

 tail of the C heir acanthus, though some- 

 what more irregularly, a rectangular 

 "/<..-.-f|Mf.-.T.- shape, is so considerably elongated, that 



Siiiijiiliii i^ assumes for its normal character as a 



'lS!!l!lJ!!jli!! scale, that of the joint of an external ray. 



A similar arrangement of external pro- 

 tection takes place in this genus over the 

 bones of the head : the cartilaginous jaws 

 receive their osseous dermal covering 

 and, with these, the hyoid bones, the 

 ]fk§gi opercules, and the cranium. And it is 



rv^ImM ^^ these dermal plates, which covered an 



*^ interior skull, of which, save in one ge- 



a. Portion of caudal ^us— the Dipterus— not a vestige re- 



fin of Cheiracan- ^ ° 



thus* mains in any of the Old Red fishes thus 



b. Portion of caudal protected, that we first trace what seem to 



jin of (Jheiroiepis '^ ' 



Cummingice. be the homologues of the cranial bones of 



(Mag. three diame- ^he osseous fishes, — at least their homo- 

 ters.) , r. 1 . 7 



logues so far as the cuticular can represent 



the internal. They appear for the first time, not as modified 

 spinous processes, broadened, as in the carpace of the Chelo- 

 nians, into osseous plates, but like those cor/ieows external plates 

 of this order of reptiles (known in one species as the tortoise- 

 shell of commerce), the origin of which is purely cuticular, and 

 which evince so little correspondence in their divisions with 

 the sutures of the bones on which they rest, that they have 

 been instanced, in their relation to the joinings beneath, as ad- 

 mirable illustrations of the cross-handing of the mechanician. 



* The darker upper patch in this figiire indicates a portion in which 

 the scales of the fins in the fossil state still retain their enamel ; the 

 lighter, a portion from which the enamel has disappeared. 



