13 
a circular bastion, ranges nearly round three 
fourths of a circle, each commanding so much 
of the horizon, that where the distinct vision 
of one eye ceases, that of the other eye be¬ 
gins, so that in the horizontal direction the 
combined range of both eyes was pano¬ 
ramic. 
“ If we compare this disposition of the 
eyes with that in the three cognate crusta¬ 
ceans, 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 requires universal 
vision, each eye is nearly hemispherical, and 
placed on a peduncle, by which it is pro¬ 
jected to the distance requisite to effect this 
purpose. 
“ In the serolis, the disposition of the eye, 
and its range of vision, are similar to those 
in the trilobite; but the summit of the eye is 
less elevated; as the flat back of this animal 
2 
