46 EXTINCT MONSTERS. 



sea ; " for it would take a long time to get to the surface for a 

 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 deptii pressure in- 

 creases, as divers know. But it appears that the ring of bony 

 plates fulfilled a yet more important ofiice, thereby enabhng 

 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 w^ould ; 

 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 



