_Group 2._ The whole macula is of a spongious or irregularly polyedric structure, showing prisms in vertical sections. Its concordance with the structure of the deteriorated cephalic eyes or with the so called border zone is complete. If a supposition may be hazarded, I think that the spongious or reticulate structure in the maculæ is their real and original state, a lower stage of development of the visual organs, out of which the prisms in the cephalic eyes have been formed, and that the prismatic lenses in their decay, as seen in the Asaphidæ, reveal to us their original state and structure, and thus, as it were, return to the primary stage in the maculæ. Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ? Barrandia. Illænus, Lichas, Trochurus, ? Platymetopus. Encrinurus also probably belongs to this group. _Group 3._ Maculæ with well developed globular lenses on the interior third of the macula. The blank part of the macula without any structure. In one instance (Chirurus spinulosus) the whole macula covered with lenses. Bronteus, Chirurus spinulosus. _Group 4._ The macula form sunk pits with smooth bottom. Structure unknown. In a few species there seems to be no less than four maculæ in two pair above each other. Dalmanites. _Group 6._ The elongate straight maculæ carrying on their innermost point from three to five diminutive, segregate ocelli situated on a clear, white patch. Proetus. As yet these ocelli have not been observed on the related genera Cyphaspis, Phaëtonides, Phillipsia and Griffithides, but they may be preliminarily ranked here in consequence of their close conformity. _Group 7._ The maculæ have in this group been reduced to a pair of tiny ocelli situated high up on the hypostoma, near its anterior margin. Phacops, Acaste.