The position of the hypostomic eyes on the ventral surface of the trilobites is not quite so abnormous, nor so isolated a feature amongst the crustaceans as might at first be supposed. Amongst the recent crustacea there is probably none, which as to the conformation of its labrum or hypostoma resembles the trilobites so closely as the species of the genus Apus, however different they may else be in all other respects. We have studied chiefly the hypostoma of Apus cancriformis. In its shape as seen from the outside and as to its outlines it reminds strongly of the trilobite hypostoma. It is nearly square (pl. VI figs. 46, 47) with rounded corners. The anterior margin is arched and in the middle it has a broad tonguelike prolongation. On the sides of the anterior margin there are two short, sharply pointed wings. The lateral margins are thick, almost as doubled, and bent forward so that a groove is formed alongside them and the large, evenly vaulted median surface of the hypostoma. These grooves are continued in a short groove parallel with the posterior margin. From this margin a narrow oblique surface is slanting backwards, as it were, doubling that margin. On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement (pl. VI fig. 49). They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. On the other hand we see a little higher up on the exterior side of the hypostoma two faintly elevated elliptic tubercles which as to their position and shape highly remind of the maculæ of the trilobites. They are hollow on the inside and we have not been able to find any peculiar structure in them. That they have any visual function is precluded through the circumstance that the whole exterior surface is densely covered with microscopic bristles (pl. VI fig. 49) which are especially remarkable along the posterior margin.