ICHTHYOSAURUS. 173 
Eijes. 
The enormous magnitude of the eye of the 
Ichthyosaurus (PI. 10, Fig. 1, 2), is among the 
most remarkable peculiarities in the structure 
of this animal. From the quantity of light ad- 
mitted in consequence of its prodigious size, it 
must have possessed very great powers of vision ; 
we have also evidence that it had both micro- 
scopic and telescopic properties. We find on 
the front of the orbital cavity in which this 
eye was lodged, a circular series of petrified thin 
bony plates, ranged around a central aperture, 
where once was placed the pupil ; the form and 
thickness of each of these plates very much re- 
sembles that of the scales of an artichoke (PI. 
10, Fig. 3). This compound circle of bony 
plates, does not occur in fishes ; but is found in 
the eyes of many birds,* as well as of Turtles, 
* The bony sclerotic of the Ichthyosaurus approaches to the 
form of the bony circle in the eye of the Golden Eagle (PI. 10, 
Fig. 5) ; one of its uses in each case being to vary the sphere of 
distinct vision, in order to descry their prey at long or short dis- 
tances. These bony plates also assist to maintain the prominent 
position of the front of the eye, which is so remarkable in birds. 
In Owls, whose nocturnal habits render distant vision impossible, 
Mr. Yarrel observes, that the bony circle (PI. 10, Fig. 4), is 
concave, and elongated forwards, so that the front of the eye 
is placed at the end of a long tube, and thus projects beyond 
the loose and downy feathers of the head ; he adds ; " The 
extent of vision enjoyed by the Falcons is probably denied 
to the Owls, but their more spherical lens and corresponding 
cornea give them an intensity better suited to the opacity 
of the medium in which they are required to exercise this 
