OF THE ASTEROLEPIS. 29 



like type : they merely serve to show that the placoids of 

 the first period possessed, like those of the existing seas, an 

 ability of secreting solid bone on their cuticular surfaces ; and 

 that, though at least such of them as have bequeathed to us 

 specimens of their dermal armature possessed it in the form 

 fiirthest removed from that of their immediate successors, the 

 ganoidal fishes, they resembled them not less in the substance 

 of which their dermoskeletal, than in that of which their 

 endoskeletal, parts were composed ; for the internal skeleton 

 in both orders, during these early ages, seems to have been 

 equally cartilaginous, and the cuticular skeleton equally osse- 

 ous. In the ichthyolitic formation 

 immediately over the Silurians, — 

 that of the Lower Old Red Sand- 

 stone, — the ganoids first appear; and 

 the members of at least one of the 

 families of the deposit, the Acanths, 

 — a family rich in genera and species, 

 — seem to have formed connecting 

 links between this second order and I 

 their placoid predecessors. They 



were covered with true scales (fiof. 4, „ t j! nr - .1 



^ ° ' a. Scales of Cheiracanth, 



a), and their free gills were protect- Microlepidotus. 



Shagreen of Spinax 

 thias. (Snout. ) 



(Mag. eight diameters.) 



ed by gill-covers : and so they must b- Shagreen of Spmax A can- 



^ ^ , . -, -, ^^*a«- (Snout.) 



be regarded as real ganoids ; but as 



the shagreen of the spotted dog-fish 

 nearly approaches, in form and character, to ganoidal scales, 

 without being really such, the scales of this family, on the 

 other hand, approached equally near, without changing their 

 nature, to the shagreen of the placoids, especially to that of 

 the spiked dog-fish (Spinax Acanihias). (Fig. 4, h.) We 

 even find on their under surfaces what seems to be an aj> 

 proximation to the characteristic footstalk. They so consi- 

 derably thicken in the middle from their edges inwards (fig. 



