246 ANATOMY OF VERTEBRATES. 



precipitated by infusion of galls, nor yields any gelatine upon 

 evaporation. 



In like manner the modifications of the dermal skeleton of 

 fishes have been viewed too exclusively in a retrospective relation 

 with the prevalent character of the skeleton of the Invertebrate 

 animals. Doubtless it is in the lowest class of Vertebrata that 

 the examples of great and exclusive developement of the exo- 

 skeleton are most numerous ; but some anatomists, in their zeal 

 to trace the serial progression of animal forms, seem to have lost 

 sight of all the vertebrate instances of the bony dermal skeleton, 

 except those presented by the ganoid and placoid fishes. He 

 must have sunk to the low conception that nature had been 

 limited to a certain allowance of the salts of lime in the formation 

 of each animal's skeleton, who could affirm that in the higher 

 Vertebrata ' the internal articulated skeleton takes all the earthy 

 matter for its consolidation,' l forgetting that the bulky Glyptodon, 

 and its diminutive congeners the Armadillos, have their internal 

 skeleton as fully developed and as completely ossified as in any 

 other mammal. The organising energies which perfect and 

 strengthen the osseous internal skeleton do not destroy nor in 

 any degree diminish the tendency to calcareous depositions on the 

 surface, when the habits and sphere of life of the Avarm-blooded 

 quadruped require a strong defensive covering from that source. 

 Neither do we find in the cold-blooded series that the endo- 

 skeleton of the alligator or scelidosaur was consolidated by a 

 minor amount of earthy matter than that of the naked frog or 

 horn-scaled lizard. 



The moment that the observations of the naturalist bring to 

 light the mode of life of any of those fishes which are said to 

 retain an unusual proportion of the external shell of the Inverte- 

 brata, we are in a condition to appreciate the adaptation of that 

 external defensive covering to such mode of life. The Sturgeons, 

 for example, were designed to be the scavengers of the great 

 rivers ; they swim low, grovel along the bottom, feeding, in 

 shoals, on the decomposing animal and vegetable substances 

 which arc hurried down with the debris of the continents drained 

 by those rapid currents ; thus they are ever busied reconverting 

 the substances, which otherwise would tend to corrupt the ocean, 

 into living organised matter. These fishes are, therefore, duly 

 weighted by a ballast of dense dermal osseous plates, not scattered 

 at random over their surface, but regularly arranged, as the 

 seaman knows how ballast should be, in orderly series along the 



1 xxvii. p. 527. 



