a consideration of the simple conical or tubular shells of the Pteropods. The shells of the latter are in few cases suitable for simple mathôÙeôÙmatôÙiôÙcal inôÙvesôÙtiôÙgaôÙtion, but nevertheless they are of very considerable interest in connection with our general problem.
The morphology of the Pteropods is by no means well understood, and in speaking of them I will assume that there are still grounds for believing (in spite of Boasã and Pelseneerãs arguments) that they are directly related to, or may at least be directly compared with, the Cephalopoda33.
The simplest shells among the Pteropods have the form of a tube, more or less cylindrical (Cuvierina), more often conical (Creseis, Clio); and this tubular shell (as we have already had occasion to remark, on p. 258), frequently tends, when it is very small and delicate, to assume the character of an unduloid. (In such a case it is more than likely that the tiny shell, or