IN THE CAUBONll-'EROUS STRATA. 270 
The vertebral column continues into the upper 
lobe of the tail, which is much longer than the 
lower lobe, and is thus adapted to sustain the 
body in an inclined position, with the head and 
mouth nearest to the bottom. 
Among existing cartilaginous Fishes, the ver- 
tebral column is prolonged into the caudal fin 
of Sturgeons and Sharks : the former of these 
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 protrusion and contraction, on putrid vege- 
tables and animal substances at the bottom ; 
hence they have constant occasion to keep their 
bodies in the same inclined position as the 
extinct fossil Fishes, whose feeble brush-like 
teeth shew that they also fed on soft substances 
in similar situations.* 
The Sharks employ their tail in another 
peculiar manner, to turn their body in order to 
bring the mouth, which is placed downwards 
beneath the head, into contact with their prey. 
We find an important provision in every animal 
to give a position of ease and activity to the head 
during the operation of feeding.^ 
* At the siege of Silistria, the Sturgeons of the Danube were 
observed to feed voraciously on the putrid bodies of the Turks 
and Russian soldiers that were cast into that river. 
+ This remarkable elongation of the superior lobe of the tail 
is found in every bony Fish of strata anterior to and including 
the Magnesian limestone ; but in strata above this limestone the 
