OF THE ASTEROLEPIS. 27 



the result of my researches, I must introduce him, in order 

 that he may be able to set out with me to the examination 

 of the Asterolepis from the same starting-point, to the Cela- 

 canth family, — indisputably one of the oldest, and not the 

 least interesting, of its order. 



So far as is^yet known, all the fishes of the earliest fossili- 

 ferous system belonged to the placoid or " broad-plated'' 

 order, — a great division of fishes, represented in the existing 

 seas by the Sharks and Rays, — animals that to an internal 

 skeleton of cartilage unite a dermal covering of points, plates, 

 or spines of enamelled bone, and have their gills fixed. The 

 dermal or cuticular bones of this order vary greatly in form, 

 according to the species or family ; in some cases they even 

 vary, according to their place, on the same individual. Those 

 button-like tubercles, for instance, with an enamelled thorn, 

 bent like a hook, growing out of the centre of each, which 

 ran down the back and tail, and stud the pectorals of the 

 thorn-back (Eaia davata), difier very much from the smaller 



thorns, with star-formed bases, which ^. 



Fig, 2. 

 roughen the other parts of the crea- 

 ture's body ; and the bony points 

 which mottle the back and sides of ^ 

 the sharks are, in most of the known 

 species, considerably more elongated 

 and prickly than the points which 

 cover their fins, belly, and snout. 

 The extreme forms, however, of the h 

 shagreen tubercle or plate seem to 

 be those of the upright prickle or a- Shar/reen of the TJiomhack 

 spine on the one hand, and of the t. Shagreen of Sphagodus,— 



Riant-laid, rhomboidal, scale-shaped « placoid of the Upper 



. Silurian.* 



plate on the other The minuter 



• From Murchison's " Silui-ian System.' 



