The specialization of _Sidneyia_ consists in the remarkable development of a highly complex claw on each of the third cephalic appendages, and in the compound tail-fin, built up of the last segment and one or more pairs of swimmerets. These two characteristics seem to preclude the possibility of deriving the eurypterids from _Sidneyia_ itself, but it seems entirely within reason that they may have been derived from another slightly less specialized member of the same order. That _Sidneyia_ is descended from any known trilobite seems highly improbable, but that it was descended from the same ancestral stock as the trilobites is, I believe, indicated by the presence of five pairs of appendages on the cephalon and trilobitic legs on the abdomen. =Molaria= and =Habelia.= Other so-called Merostomata found by Walcott in the Middle Cambrian are the genera _Molaria_ and _Habelia_, both referred to the Cambrian family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the Upper Cambrian in having a trilobite-like cephalon without facial sutures, a trilobite-like thorax of a small but variable (7-12) number of segments, and a _Limulus_-like telson. Neither of them has yet been fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_ appears to have five pairs of cephalic appendages, the first two pairs of which are multisegmented antennæ. The thoracic appendages are likewise none too well known, but they appear to have been biramous. The endopodites are better preserved than the exopodites, but in at least one specimen of _Molaria_ the exopodites are conspicuous. If these genera are properly described and figured, their appendages are typically crustacean, and fundamentally in agreement with those of _Marrella_. The relation to the Trilobita is evidently close, the principal differences being the absence of facial sutures and the presence of true antennæ. I am therefore transferring the Aglaspidæ from the Merostomata to a new subclass under the Crustacea. ARANEÆ. The spiders have the head and thorax fused, the abdomen unsegmented except in the most primitive suborder, and so appear even less trilobite-like than the insects. The appendages likewise are highly specialized. The cephalothorax bears six pairs of appendages, the first of which are the pre-oral cheliceræ, while behind the mouth are the pedipalpi and four pairs of ambulatory legs. The posterior pairs of walking legs belong to the thorax, but the anterior ones are to be homologized with the maxillæ of Crustacea, so that the spiders are like the trilobites in having functional walking legs on the head.