30 Remarks on the Trilohiie. 



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 distinct 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 purpose. 



" In the scrolls, 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, scrolls, 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 scrolls. 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, 



* See annexed the plates of Dr. Buckland, which being important to the just 

 comprehension of the subject, we have caused to be copied. — Eds. 



