]4-8 LECTURE VI. 



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 afiirm that in the 

 higher Yertebrata "the internal articulated skeleton takes all the 

 earthy matter for its consolidation" (xxvii. p. 537.), forgetting that 

 the bulky Griyptodon and its diminutive congeners the Armadillos have 

 their internal skeleton as fully developed and as completely ossified 

 as in any other mammals. 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 warm-blooded quadruped 

 require a strong defensive covering from that source. 



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 Invertebrata, 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 ani- 

 mal and vegetable substances which are hurried down with the debris 

 of the continents drained by those rapid currents : thus they are ever 

 busied re-converting 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 middle and at the sides of the body. The protection against the 

 water-logged timber and stones hurried along their feeding grounds, 

 which the Sturgeons derive from their scale-armour, renders needless 

 the ossification of the cartilaginous case of the brain or other parts 

 of the endo-skeleton : and the weight of the armour requires that 

 endo-skeleton to be kept as light as may be compatible Avith its elastic 

 pi'operty and other functions. The Sturgeons are further adjusted to 

 their place in the liquid element, and endowed with the power of 

 changing their level and rising with their defensive load to the 

 surface, by a large expansive air-bladder. 



These teleological interpretations of the dermal bony plates may 

 give some insight into the habits and conditions of existence of those 

 Ganoid and heavily protected Placoid fishes which so predominated 

 in the earlier periods of animal life in our planet ; whereas these 

 Ganoids and Placoids have hitherto been viewed almost exclusively by 

 the light of the analogy of an embryonic " Age of Fishes," or explained 



