These are found solely in the family of the Phacopidæ, unless the Lichadidæ were also provided with this sort of eyes, but we have had no opportunity to study them. It seems, however, not likely that they had aggregate eyes. BARRANDE has represented them quite as finely reticulate as the eyes of any Asaphid. We have sectioned and figured the eyes of Dalmanites vulgaris and D. obtusus from the Silurian of Gotland and found that these have truly aggregate eyes, each consisting of a regular biconvex lens, lying enclosed in a socket of its own and covered by a cornea of its own. The distance between the eyes is much variable and in a few instances they are nearly contiguous. Extremes are seen on pl. III figures 43, 47. The lenses are comparatively large, and have always had a covering membrane, though this in many instances has been lost. This membrane which is an immediate continuation of the general integument of the body covers the lenses all round their superior moiety. In its prolongation downwards between the lenses (Pl. VI fig. 3, 4) it is free from the contact with them and hangs alongside and around much incrassated, so as to take in a section a lengthened lancet like shape. It lies thus alongside the other interstitial test, and is like this perforated by longitudinal canals. In a horizontal section taken a little below the surface it encircles the lens as a wall like ring (Pl. VI fig. 1, 2). In a vertical section the lenses lie in direct contact with the cheek without any intervening zone and the cheek has the structure so common amongst the trilobites, being perforated by vertical tubes going straight down from the surface (Pl. III fig. 44, Pl. VI f. 5).