OF THE EARLIER VERTEBRATA. 39 



cervical or dorsal appendage. In the wide range of the ani- 

 mal kingdom, there are scarce any two pieces of organization 

 that less resemble each other in form than the vertebrae of 

 the placoids resemble their skulls ; and the dij[fereiice is not 

 merely external, but extends to even their internal construc- 

 tion. In both skull and vertebrae we detect an union of 

 bone and cartilage ; but the bone of each vertebra forms an 

 internal continuous nucleus, round which the cartilage is 

 arranged; whereas in the skulls it is the cartilage that is 

 internal, and the bone is spread in granular points over it. 

 If we dip the body of one of the dorsal vertebrae of a herring 

 into melted wax, and then withdraw it, we will find it to re- 

 present in its crusted state the vertebral centrum of a pla- 

 coid, — soft without and osseous within ; but in order to re- 

 present the placoid skull, we would have first to mould it 

 out of one unbroken piece of wax, and then to cover it over 

 with a priming of bone-dust. And such is the efiect of this 

 arrangement, that, while the skull of a placoid exposed to 

 a red heat falls into dust, from the circumstance that the 

 supporting framework on which the granular bone was ar- 

 ranged perishes in the fire, the vertebral centrum, whose in- 

 ternal framework is itself bone, and so not perishable, comes 

 out in a state of beautiful entireness, — resembling in the 

 thornback a squat sand-glass, elegantly fenced round by the 

 lateral pillars (fig. 8, h) ; and in the dog- ^. ^ g 



fish (a) a more elongated sand-glass, in 

 which the lateral pillars are wanting. 

 Such are the heads and vertebral joints 

 of the existing placoids; and such, rea- ^ ^ 



soning from analogy, seem to have been a. Osseous centrum of 

 the character and construction of the ^ %TeomieZu^of 

 heads and vertebral joints of the pi a- R<^i(^ clavata. 



coids of the Silurian period. (^^*- "^^^'^ 



The most ancient brain-bearinij craniums that have come 



