When LILJEVALL saw this group of granules he was at once struck with the perfect, outward similarity between them and the facets of the compound eyes on the upper surface of the head of this same species, and I fully concurred in the same view. It now became of importance to ascertain whether the intimate structure also corresponded in both, and microscopic sections were accordingly prepared. Although not so clear as in other species of this genus a vertical section of a cephalic eye (Pl. II fig. 21) shows a median row of black spots which compared with other sections (Pl. II fig. 9) must be the centres of the badly preserved lenses. This is confirmed by the horizontal section (Pl. II fig. 20) where the well known image of the polyedric facets appeared. In the same manner the granulated spot of the hypostoma shows in a vertical section even more clearly (pl. II fig. 27) the elongate, coherent lenses with a black centre and the polyedric form of the lenses in a horizontal section (Pl. II fig. 26). The assumption of a pair of small adventive eyes on the exterior side of the hypostoma became thus based on the clear evidence of the perfect structural agreement between them and the eyes of the head. But it was necessary to strengthen this evidence through extended researches on the hypostoma of as many trilobite genera as possible and also to study the intimate structure of the cephalic eyes in as many genera as could be accessible. We shall now give first a general account of the structure of the eyes in the trilobites as far as we have been able to study them, and then proceed to describe the maculæ of the various genera, which we have observed. With regard to the presence or non existence of visual organs, we must remark that a considerably greater number of genera than those which unanimously have been regarded as blind, also are devoid of eyes though they still by many authors are ranked amongst the oculate ones and we then had better first to make a review of them and thus to eliminate them from the number of the oculate species. =Blind Trilobites.= In the chaos of generic forms and in the great disagreement which prevails as to the systematizing of the trilobites of the Cambrian time, there is a thorough revision of them highly needed by a person having access not only to the literature, but also to the original specimens. It is almost impossible in the present state of things to tell with any degree of certainty how many well established genera had been living during that period. Hence the difficulty of fixing the systematic names of many specimens the visual organs of which are to be described.