30 Remarks on the Trilobite. 
ion in every direction the contrivance is complete. The form of 
each eye is nearly that of the frustrum 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 vis- 
ion was required. The exterior of each eye, like a circular bas- 
tion, ranges nearly round three fourths of a circle, each command- 
ing so much of the horizon, that where the dintings vision of one 
eye ceases, that of the other eye begins, so that in the horizontal 
direction the combined range of both eyes was panoramic. 
“If we compare this disposition of the eyes with that in the 
three cognate crustaceans,* by which we have been illustrating 
the general structure of the trilobites, we shall find the same 
mechanism pervading them all, modified by peculiar adaptations 
to the state and habits of each; thus in the branchipus, which 
moves with rapidity in all directions through the water, and re- 
quires universal vision, each eye is nearly hemispherical, and 
placed on a peduncle, by which it is projected to the distance re- 
quisite to effect this pur 
“Tn the serolis, the disposition of the eye, and its range of vis- 
ion, are similar to those in the trilobite, but the summit of the eye~ 
is less elevated ; as the flat back of this animal presents little ob- 
struction to the rays of light from surrounding objects. 
“In the limulus, where the side eyes are sessile, and do not 
command the space immediately before the head, two other sim- 
ple 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 com- 
plex organ, the eye, selected from each extreme, and from a mid- 
way 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 chronology, when 
the secondary strata were deposited at the bottom of a warm sea, 
5 coil aa all 
exed the plates of Dr. Buckland, which being eeertant to the just 
comprehension of the subject, we have caused to be copied.—Eps 
ii 
a ais a 
