300 M. F. Dreyer on the The last o£ them, which bears the furca, is also unsymmetri-cally developed. In correspondence with the relative sizes of the two halves of the furca, the left side of the segment in question is considerably broader than the right. The serra-tion characteristic of the furca of this species terminates regu-larly on each side just above the lateral seta, but on the right side it commences, not as on the left immediately beneath the hinder margin of the last segment, but somewdiat further back. This malformation may be original, but is more probably pro-duced by a subsequent accident (perhaps in change of skin). In the reproduction of the lost part of the furca it was not again completed of tlie normal size ; but in this way certainly the displacement of the above-mentioned seta is not easy to explain. XL. — Considerations on the Structure of Rhizopod Shells. By FiiiEDRiCH Dreyer *. In the course of my investigations upon Rhizopoda, and especially upon the Hadiolaria, various considerations of a general character have impressed themselves upon me. In part similar ideas have been already touched upon by previous authors and occur scattered in the most various parts of the copious literature ; several points to be referred to in what follows I have already incorporated with the special investi-gations in the first part of my ' liadiolarienstudien ' f ; never-theless I regard it as a not unprofitable task to reproduce in the following pages in a connected torm the complete train of thought of my considerations upon the structure of the Rhizo-pod shells, as I hope that it will be of interest even for many who do not occupy themselves specially with the Protistan group in question. Even on a superficial consideration of the enormous num-ber of forms of the Rhizopoda we may recognize in them an essential difference in the general habit of the shell, and, in accordance therewith, distinguish two groups of forms. One * Translated from the ' Biologisches Centralblatt/ Bd. ix. pp. 333-352 (1st August, 1889). t F. Dreyer, ' Morphologische Radiolarienstudien,' Heft I. " Die I'ylombildungen in vergleicheud-auatomischer und entwicklungsgescbicht-licher Beziehuug bei Kadiolarieu uud Protisten iiberbaupt, oebst System uud Bescbreibuug neuer uud der bis jetzt bekannten pylomatischen Spu-mellarien" (Jeua, 1880).