14 
presents little obstruction to the rays of light 
from surrounding objects* 
“ In the limulus, where the side eyes are 
sessile, and do not command the space imme¬ 
diately before the head, two other simple eyes 
are fixed in front, compensating for the want 
of range in the compound eyes over objects 
in that direction. 
“ In the above comparison of the eyes of 
trilobites, with those of the limulus, serolis, 
and branchipus, we have placed side by side, 
examples of the construction of that most 
delicate and complex organ the eye, selected 
from each extreme, and from a midway place 
in the progressive series of animal creations. 
We find in trilobites of the transition rocks, 
which were among the most ancient forms of 
animal life, the same modifications of this 
organ which are at the present time adapted 
to similar functions in the living serolis. The 
same kind of instrument was also employed 
in those middle periods of geological chrono¬ 
logy when the secondary strata were deposit¬ 
ed at the bottom of a warm sea, inhabited by 
