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Rh envelope, are presented by the genera Trichodiscus, Plagiophrys, and Euglypha, which are associated with it by MM. Claparède and Lachmann in the family Actinophryna. But they seem to me—so far as I can judge by the published descriptions of these animals, which I have not myself bad the opportunity of examining in their living state —not less unmistakeably exhibited by the Acanthometrina and the Polycystina, which may be regarded as higher or more specialized forms of the same type. The radiating pseudopodia of Acanthometra correspond precisely in all their characters with those of Actinophrys; having the same rod-like tapering form, the same regularly radiating arrangement, the same mutual isolation, and the same slow movement of particles along their surface: some of them, however, are enclosed in tubular siliceous sheaths, which appear to be secreted from their surface; and the union of the expanded bases of these sheaths forms a sort of framework, that supports the protoplasmic substance of the body. In this substance the differentiation of endosarc and ectosarc has obviously proceeded further than in Actinophrys; and the endosarc contains a number of cell-like bodies resembling those of the Thalassicollina. The animal of the Polycystina seems to correspond with Acanthometra in all essential particulars, the difference being only in the disposition of the siliceous envelope; and that of the Thalassicollina appears to be only a more composite aggregation of the like structural components. For details of the evidence of the relations of the last-named groups to each other and to the preceding, I must refer to the memoir of Prof. Müller already cited; and his designation I adopt as that of the group to which he applied it, with the addition of the family Actinophryna. That family, as I have endeavoured to show, really supplies the typical form of the Order; the naked Actinophrys bearing the same relation to the testaceous Polycystina (for example) that the naked Amœba does to the testaceous Arcella and Difflugia, or the naked Lieberkühnia to the testaceous Gromida and Foraminifera.

III.—From the Actinophryna and the other Rhizopods of the order Radiolaria, the Amœbina seem to me to be very definitely distinguished by the more complete differentiation of the containing and the contained portion of their sarcode-bodies. and by the entire difference (as regards, at least, the typical forms of each group) in the character of their pseudopodial extensions. The distinction between the ectosarc and the endosarc is far more clearly marked in Amœba than in Actinophrys; the latter being much more fluid, whilst the consistence of the former is much firmer. It is through the endosarc alone that those coloured and granular particles are