There are certainly no maculæ of the same sort as in so many other trilobites and the interior side of the hypostoma does not bear the least traces of impressions which might be taken as the reverse of the maculæ and still more less as muscular impressions. In fact, the total absence of such in Acidaspis militates against the interpretation of the macula impressions as muscular scars. When the maculæ fail, also the so called muscular impressions fail. Moreover we are able to see still clearer in this matter through what we know about the structure and position of the extremely similar hypostoma of Apus, of which we have given a description in the end of this memoir. This skeletal part of Apus is without any connection with the surroundings, excepting at its anterior margin, and is consequently movable in a direction outwards and upwards. Along that anterior margin it is fixed to the outside of the ventricle through three pair of muscles, three muscles on each side of the margin. ZADDACH De Apodis cancriformis anatome (pl. II fig. XIV p. 68). For the rest the marks of the attachment of the muscles are as a rule in the Crustacea, at least those of the head, elevated small platforms, so in the Trilobites, of which I have excellent specimens in a Bumastus and others. On the inside of the head of Limulus they are faintly elevated patches. The small hollows on the inside of the hypostoma formed by the maculæ are indeed the sockets in which the soft parts of these more or less developed hypostomic eyes were sheltered. But there is still a feature in the hypostoma of Acidaspis which merits our attention and which perhaps may have a significance akin to that of the maculæ.