The hypostoma is square, with two short pointed wings, one on each side of the slightly bent anterior margin and likewise two smaller ones at the corners of the posterior margin. A groove follows on a short distance the lateral margins and the posterior margin and disappears a little below the anterior one. In the same direction, distally, though a little more inside and unconnected with them there are two small grooves, the bottom of which consists of a shell substance of different colour and structure than the other parts. Having been a little ground and seen in transmitted light it exhibits the shape of a club and a homogenous yellow spot, tapering posteriorly and swelling out distally (Pl. I fig. 4). It must be left an open question whether these maculæ share in the nature of visual organs as the quite different maculæ of the other trilobites, but it may be possible that it is so. It must, however, be remarked that there are two types of hypostoma in the genus Acidaspis as shown by the illustrations of BARRANDE. One has the small grooves, possibly all sheltering the claviform maculæ, disposed as in the now described A. crenata. This group embraces five species of the Bohemian Silurian formation. The other group of three species again has the hypostoma of the same quadratic or rectangular shape, but the two short grooves, which may be expected to contain the maculæ, are placed midways between the anterior and the posterior margins, nearly as the maculæ bearing grooves of other trilobites. We have however not had material for pursuing our researches in this genus, the other species of the Swedish Acidaspidæ being unknown as to their hypostoma. It may here be added an observation concerning the ornamentation of the exterior surface of the hypostoma of Ac. crenata. It is covered by a great number of diminutive circular or oblong wartlets occupying the whole surface excepting the lower third of the central field just above the posterior groove which is smooth. These wartlets seen through transmitted light (Pl. I fig. 5) show in their interior something like a peculiar black spiculum rising from a bifid rootlet and confined within the wartlet and not extruding from it. In a longitudinal section (Pl. I fig. 6) the spicula perforate the wartlets reaching through their whole length. As in Calymmene these interior pseudo-spicula are tubes, filled with iron-pyrites. It is probable that these tubes were once bearing setæ and quite as in Apus formed a fur of bristles. The structure of the cephalic eyes (Pl. 1 f. 1-2) is prismatic, but the separate prisms are rather short and broad. Their lower or interior end is convex. The separating lines between the single prisms are not always distinct. =Agnostus= DALM. Pl. 1 fig. 7.