220 
DRAGONS OF THE PRIME. 
Oct., 1891 . 
endure, the lower jaw (Fig.5) is made, not of a single bone, hut 
of six separate pieces on each side, which are braced and united 
together so as to form a structure which is at once light and 
strong, and gives the greatest elasticity with the least weight. 
The same principle is involved as when several separate 
pieces of steel are united to form a spring for a carriage. 
Another feature of the head of interest in regard to the 
mode of life of Ichthyosaurus was the enormous size of the eves. 
(Fig. 4.) In the largest specimens they were as much as a foot 
across, and each was protected by a ring of bony pieces ( Br .). 
Now very large eyes are found in those fishes which live at 
such great depths in the sea that it is almost quite dark, the 
rays of sunlight which penetrate there being so few and feeble, 
Hence, we may perhaps conclude that Ichthyosaurus also went 
down into the abysses of the ocean, where he needed his 
enormous eyes to catch the few and dim rays of light which 
penetrated so far. 
As to the bony ring, that had two uses ; in the first place, 
it served to protect the eye from the pressure of the water at 
great depths, and from the force of the waves at the surface ; 
in the second, it was the point of attachment of the great 
muscles, which altered the shape of the lens in accordance 
with the need for long distance or short distance vision. Thus, 
by means of the ring, the eye could be converted into a 
microscope for near vision, and into a telescope for distant 
vision, and was, therefore, an optical instrument of varied 
and great power, enabling the Ichthyosaurus to descry its 
prey at great or little distances in the obscurity of night and 
of the depths of the sea. 
But the Ichthyosaurus had not only the two ordinary 
eyes, one on each side of the head, he also had an eye on the 
top of his head in the middle. To prove this statement now, 
would take me too far away from my present subject. I will 
only say that in the top of the skull of Ichthyosaurus is a 
great hole, through which light entered into the third eye 
that was seated near to the surface of the brain. 
So much for the adaptations of structure to the mode of 
life : now, with regard to those features which are of interest 
as throwing light on the pedigree of Ichthyosaurus and its 
relations to other animals. Undeniably, Ichthyosaurus is a 
reptile. The structure of its skull and teeth, as well as 
other features of its skeleton, establish this beyond a doubt. 
Yet, though a reptile, it is not closely related to any other 
known, either living or extinct, while in some features of its 
structure it resembles fishes and amphibia. 
(1.) In the first place, its vertebra are not like those of most 
