46 EXTINCT MONSTERS. 
sea ;” for it would take a long time to get to the surface fora 
fresh supply of air. 
Perhaps no part of the skeleton is more interesting than the 
curious circular series of bony plates surrounding the iris and pupil 
of the eye. The eyes of many fishes are defended by a bony 
covering consisting of two pieces ; but a circle of bony overlapping 
plates is now only found in the eyes of turtles, tortoises, lizards, and 
birds, and some alligators. This elaborate apparatus must have 
been of some special use; the question is—What service or 
services did it perform? Here, again, we find answers suggested 
by Owen and Buckland. It would aid, they say, in protecting the 
eye-ball from the waves of the sea when the creature rose to the 
surface, as well as from the pressure of the water when it dived 
down to the bottom—for even at a slight depth pressure in- 
creases, as divers know. But it appears that the ring of bony 
plates fulfilled a yet more important office, thereby enabling 
the fish-lizards to play admirably their part in the world in which 
they lived, and to succeed in the struggle of life; for even in 
those remote days there must have been, as now, a keen competi- 
tion among all animals, so that the victory was to those that were 
best equipped. 
Would it not be an advantage for them to have the power of 
seeing their finny prey whether near or far? Certainly it would; 
and so we are told that, by bringing the plates a little nearer 
together, and causing them to press gently on the eye-ball, so as 
to make the eye more convex—that is, bulging out—a nearer 
object would be the better discerned. On the other hand, by 
relaxing this pressure, thus enlarging the aperture of the pupil 
and diminishing the convexity, a distant object would be focussed 
upon the retina. In this manner some birds alter the focus of 
their eyes while swooping down on their prey. 
What a wonderful arrangement! We often hear of people having 
two pairs of spectacles—with lenses of different curvature—one 
for reading, and the other for seeing more distant objects than a 
