The interior surface of the hypostoma shows all the protuberances of the exterior surface (tubercular maculæ, spines and granulations) as shallow pits, the exterior grooves being on the contrary elevated ridges. For the rest the interior surface is smooth and its margins are posteriorly covered with a more or less broad duplicature, from the lateral margins of which the posterior wings project. The manner in which LILJEVALL became aware of the interesting structural features of which I am going to give an account, is as follows. We were about to describe and delineate some new or not sufficiently known Upper Silurian trilobites from the island of Gotland. In order to draw a specimen of Bronteus polyactin ANGELIN, which is rarely found in the Wenlock and Ludlow strata of Gotland, LILJEVALL was preparing and cleaning its hypostoma. The shape of this hypostoma, as represented in pl. II fig. 22, is clypeiform, tapering towards the posterior margin, which is regularly tongue shaped. The anterior margin is faintly curved and at both sides elongated in a broad, pointed wing. Its exterior surface is divided in three transverse fields, defined through two shallow grooves, the anterior and largest field occupying more than the moiety, the median one is crescent-shaped and narrow, and the posterior one is nearly of the same size and confluent with the lateral borders of the hypostoma, which near the median transverse axis of the latter project in a blunt angle on each side. The surface, for the rest smooth, is covered by concentric, irregular terrace lines. On the inferior edge of the upper groove two small tubercles are situated, one on each side closer to the lateral margins than to the central axis. The size of their longitudinal axis amounts to 0,96 mm. in the shortest and to 1,07 mm. in the longest specimens. This axis is parallel with the groove and consequently oblique to the longitudinal axis of the hypostoma. They are oblong or ellipsoid, their inferior apices bluntly pointed or rounded, varying a little as seen in the three specimens figured, (Pl. II figs. 23, 24, 25). For about two thirds their surface is perfectly smooth or rather glossy, and the lower third is covered with a compact accumulation of small granules. The extension of this peculiar accumulation varies, being somewhat larger in some specimens, and the limit which runs oblique towards the glossy part is more or less curved. Such a granule taken single is of the extremest minuteness, measuring 0,055 millimeter at the highest. Their outline is perfectly circular and semiglobose.