Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as essentially the same, and accounted for the conspicuous lenses of Phacops on the supposition that the cornea was thinner in that genus than in the trilobites with smooth eyes. Barrande (1852, p. 135) recognized three types of eyes in trilobites, adding to Quenstedt's smooth and facetted compound eyes the groups of simple eyes found in Harpes. In his sections of 1852, pl. 3, figs. 15-25, which are evidently diagrammatic, he shows separated biconvex lenses in both types of compound eyes, _Phacops_ and _Dalmanites_ on one hand, and _Asaphus_, _Goldius_, _Acidaspis_, and _Cyclopyge_ on the other. Clarke ( 1888), Exner ( 1891 ) and especially Lindstroem (1901) have since published much more accurate figures and descriptions. The first person to study the eye in thin section seems to have been Packard (1880), who published some very sketchy figures of specimens loaned him by Walcott. He studied the eyes of _Isotelus gigas_, _Bathyurus longispinus_, _Calymene_, and _Phacops_, and decided that the two types of eyes were fundamentally the same. He also compared them with the eyes of _Limulus_. Clarke (1888), in a careful study of the eye of _Phacops rana_, found that the lenses were unequally biconvex, the curvature greater on the inner surface. The lens had a circular opening on the inner side, leading into a small pear-shaped cavity. The individual lenses were quite distinct from one another, and separated by a continuation of the test of the cheek. Exner (1891, p. 34), in a comparison of the eyes of Phacops and _Limulus_, came to the opinion that they were very unlike, and that the former were really aggregates of simple eyes. Lindstroem (1901, pp. 27-31) came to the conclusion that besides the blind trilobites there were trilobites with two kinds of compound eyes, trilobites with aggregate eyes, and trilobites with stemmata and ocelli. His views may be briefly summarized. I. Compound eyes. 1. Eyes with prismatic, plano-convex lenses. "A pellucid, smooth and glossy integument, a direct continuation of the common test of the body, covers the corneal lenses, quite as is the case in so many of the recent Crustacea. The lenses are closely packed, minute, usually hexagonal in outline, flat on the outer and convex on the inner surface. Such eyes are best developed in _Asaphus_, _Illænus_, _Nileus_, _Bumastus_, _Proëtus_, etc." 2. Eyes with biconvex lenses. The surface of the eye is a mass of contiguous lenses, covered by a thin membrane which is frequently absent from the specimens, due to poor preservation. The lenses are biconvex, and being in contact with one another, are usually hexagonal, although in some cases they nearly retain their globular shape. Such eyes are found in Bury care, _Peltura_, _Sphæropthalmus_, _Ctenopyge_, _Goldius_, _Cheirurus_, and probably others. II. Aggregate eyes.