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vision is greater or smaller as the exterior of 
the eye is more or less hemispherical. 
“ If we examine the eyes of trilobites with 
a view to their principles of construction, we 
find both in their form, and in the disposition 
of the facets, obvious examples of optical 
adaptation. 
“ In the asaphus caudatus each eye con¬ 
tains at least 400 nearly spherical lenses fixed 
in separate compartments on the surface of 
the cornea. The form of the general cornea 
is peculiarly adapted to the uses of an animal 
destined to live at the bottom of the water: 
to look downwards was as much impossible 
as it was unnecessary to a creature living at 
the bottom; but for horizontal vision in every 
direction the contrivance is complete. The 
form of each eye is nearly that of the frustum 
of a cone, incomplete on that side only which 
is directly opposite to the corresponding side 
of the other eye, and in which, if facets were 
present, their chief range would be towards 
each other across the head, where no vision 
was required. The exterior of each eye, like 
