60 THE OLD RED SANDSTONE. 
which exist between the digestive organs and the hoofs of the 
ruminant animals; but it is of importance that they should 
be noted.* It may be remarked, further, that the great bulk 
of fishes whose skeletons consist of cartilage have yet an 
ability of secreting the calcareous earth which composes bone, 
and that they are furnished with bony coverings, either par- 
tial or entire. Their bones lie outside. The thorn-back de- 
rives its name from the multitudinous hooks and spikes of 
bone that bristle over its body; the head, back, and opercu- 
lum of the sturgeon are covered with bony plates; the thorns 
and prickles of the shark are composed of the same mate- 
rial. The framework within is a framework of mere anima. 
matter; but it was no lack of the osseous ingredient that led 
to the arrangement—an arrangement which we can alone 
refer to the will of that all-potent Creator, who can transpose 
his materials at pleasure, without interfering with the perfec- 
tion of his work. It is a curious enough circumstance, that 
some of the osseous fishes, as if entirely to reverse the con- 
dition of the cartilaginous ones, are partially covered with 
* Dr. Buckland, in his Bridgewater Treatise, assigns satisfactory 
reasons for this construction of tail in sharks and sturgeons. Of the 
fishes of these two orders, he states, ‘the former perform the office 
of scavengers, to clear the water of impurities, and have no teeth, 
but feed, by means of a soft, leather-like mouth, capable of protru- 
sion and contraction, on putrid vegetables and animal substances at 
the bottom; and hence they have constantly to keep their bodies in 
an inclined position. The sharks employ their tail in another pecu- 
liar manner—to turn their body, in order to bring their mouth, 
which is placed downwards beneath the head, into contact with their 
prey. We find an important provision in every animal, to give a po- 
sition of ease and activity to the head during the operation of feed- 
ing.” — Bridgewater Treatise, p. 279, vol. i., first ed. 
