SHARKS, DOG-FISHES, SKATES AND RAYS 43 
These rings assist in concealing the sharp-cut free edge of 
the fin among the round stones on the bottom. The ray 
under consideration is the same fish as that shown on 
the plate facing p. 44. Here several rings are again 
clearly seen on the edge of the fins, but if the ground 
is examined other rings of a similar appearance will be 
detected. These are due to the stones on the bottom, 
for when looked down upon from above the stone forms 
the centre of the ring, and the shadow itself the dark 
ring round it. Similar rings can easily be seen if round 
stones are dropped into a deep tub so that the light 
falls on them only from above. 
Sharks and rays are classed together, and it seems 
strange that fish so very different in their shape and 
habits should be so nearly related ; but this dissimilarity 
in external appearances in nearly-related species is con- 
stantly met with. Sharks and rays are among the most 
ancient of fishes, and probably had a common ancestor. 
Some of these took to active life, chasing and devouring 
swimming fishes, and hence they became elongated in 
shape, and developed powerful tails. Others became 
bottom feeders, and by a gradual process of evolution, 
extending over countless ages, they became flat, and 
as their habits of feeding do not necessitate rapid move- 
ment, the tail as a swimming organ became functionless. 
The manner in which rays are flattened is of interest, 
since it differs from the method of flattening of the 
so-called “‘ flat fishes,’ such as the plaice and the turbot. 
The ray is flattened from above downwards, and 
