"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.
The individual lenses are comparatively large, distinct from one another, each lying in its own socket. There is, however, a thin membrane, which covers all those in any one aggregate, and is a continuation of the general integument of the body. This membrane is continued as a thickened infolding which forms the sockets of the lenses.
Such eyes are known in the Phacopidæ only.
III. Stemmata and ocelli.
The stemmata are present only in Harpes, where there may be on the summit of the cheek two or three ocelli lying near one another. Each, viewed from above, is nearly circular in outline, almost hemispheric, glossy and shining. In section they prove to be convex above and flat or slightly concave beneath. The test covers and separates them, as in the case of the aggregate eyes.
The ocelli of the Trinucleidæ and Eoharpes are smaller, and the detailed structure not yet investigated.
Lindstroem concludes that so far as its facets or lenses are concerned, the eye of the trilobite shows the greatest analogy with the Isopoda, and the least with Limulus.