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Show 1881.] ON SOCOTRAN LAND-SHELLS. 801 10. O n the Land-Shells of the Island of Socotra collected by Prof. I. Bayley Balfour. By Lieut.-Colonel H . H. G O D W I N - A U S T E N , F.R.S., F.Z.S., & c - P a r t II. Heli-cacese1. (Plates LXVIII., LXIX.) The most abundant Pulmoniferous shells on the island of Socotra are those of a subgenus of Buliminus, which Mr. Geoffrey Nevill separated and distinguished under the title Achatinelloides, upon the then only known species B. socotrensis of Pfeiffer, one of the most distinct forms of the group. Achatinelloides (which I propose for the present to restrict to species of this island until we possess a larger series of the land-shells of the adjacent mainland, almost unknown at present) may be divided into two well-marked divisions:-a, with well costulated sculpture; B, smooth and polished. A broad, rather flat columellar margin, more or less plicate, is characteristic of the subgenus, and is well shown in the type (vide fig. 1 a, Plate LXVIII.). There is a third group of Bulimini peculiar to Socotra, c, characterized by their thin fragile shells, while the columellar margin still assimilates to that of the more solid shells of the first group (compare fig. 5 a, Plate LXVIII., with fig. 9 a, Plate LXIX.). As yet we know nothing of their anatomy; but as they approach in form and coloration B. velutinus, Pfr., which is the type of Albers's subgenus Pachnodus, I have placed them in that subgenus for the present. Groups a and B show, in a very interesting manner, how gradually species have been modified, and yet how close they remain as a whole. Take, for instance, the markings in A. socotrensis and socotrensis var. elongatus, and observe how the spiral markings in the former have become longitudinal on the sides of the longer whorls of the latter, and how these last again have been modified in A. tigris into still more pronounced longitudinal rays of colour, while the shell has changed into the more elongate form and lost its ribbed sculpture. Again, in A. zeBrinus we have another modification of the same spiral bands seen in A. socotrensis, but in a still more solid glossy shell. In A. Balfouri, in the first 4 or 5 whorls only do we find any coloured bands retained; these, as the shell becomes mature, are lost altogether. Plain unmarked varieties occur in many of the species. A. hadiBuensis has the coloured patches very irregularly arranged, but still oblique to the costulation ; while in A. gollonsirensis, the most ornamented of all, this type of coloration is crossed by a well-marked almost continuous spiral band of colour. A. semicastaneus presents in its form and coloration quite another sort of variation in another direction. In bringing the list of Socotran Land-shells to a conclusion in this paper, I think it due (as a member of the Committee for the exploration of the island) to express to Prof. Bayley Balfour how much we are 1 For Part I. vide supra p. 251 et seqa. PROC. ZOOL. Soc-1881, No. LII. 52 |