We find the maculæ of the trilobites present from the oldest Cambrian times and we find also in them a progressive evolution, in some to a high degree, lenses and facets, perfectly identical with those of the eyes on the head shield, converting them into true eyes. It was in the Lower Silurian times, in the Lower grey Orthoceratite limestone of Sweden, that Illænus Esmarki appeared provided with actual prismatic lenses on its maculæ and probably also Lichas. Still higher at the top of the Lower Silurians, in the Leptæna limestone of Dalecarlia the genus Bronteus has appeared with those highly developed hypostomic eyes. But there are, no doubt, still more facts to adduce for filling up extant lacunæ in the knowledge of these matters. The material now at hand has, however, not been sufficient for doing this. At present I must leave this interesting field of investigation and be contented if I have succeeded in showing where rich harvests await every one who will make a search for them. Explanation of the plates. All specimens figured belong to the Swedish State Museum, unless otherwise stated. Pl. I. =Acidaspis crenata= EMMR. p. 85. Fig. 1 & 2. Vertical sections of the eye 60/1. » 3. The outside of the hypostoma 6/1. » 4. The anterior margin of the hypostoma, showing one of the clubshaped spots in transmitted light 20/1. » 5. The granulations of the hypostoma in transmitted light 80/1. » 6. Vertical section of the hypostoma with granules 60/1. Specimens from Djupvik, Eksta. =Agnostus glandiformis= ANGELIN p. 35. Fig. 7. Vertical section of a part of a rolled up specimen 8/1. Specimen signed CQ. Andrarum, Scania. =Asaphus expansus= L. p. 39. Fig. 8. Horizontal section of the eye near the upper margin. _a._ The surrounding spongious zone, _b._ The surface of the eye with lenses 60/1. Spec. CG. » 9. Horizontal section from the centrum of the eye 60/1. Spec. CF. » 10. Horizontal section near the inferior margin, _a_ the eye, _b_ marginal zone, _c_ a portion of the cheek 60/1. Specim. CF. » 11. Vertical section near the inferior margin 60/1, same specimen as in fig. 10. » 12. Eye fractured vertically showing regular prisms in the eye (_a_) and irregular ones in the marginal zone (_b_) 30/1. » 13. Part of the hypostoma with the macula 12/1. » 14. The right macula from the inside 12/1. » 15. A vertical section across the hypostoma, _aa_ the maculæ 8/1. » 16. A similar one of the macula at left in the preceding figure 60/1. » 17. The right macula from the outside in transmitted light 60/1. Sp. BQ. =Asaphus fallax= DALM.? p. 41.