1881.] ON SOCOTftAN LAND-SHELLS. 801 



10. On the Land- Shells of the Island of Socotra collected 

 by Prof. I. Bayley Balfour. By Lieut.-Colonel H. H. 

 GoDwiN-AusTEN, F.R.S., F.Z.S., &c.— Part II. Heli- 



cacese*. 



(Plates LXVIII., LXIX.) 



The most abundant Pulmouiferous shells on the island of Socotra are 

 those of a subgenus o( Buliminus, which Mr. Geoffrey Nevill separated 

 and distinguished under the title Achatmelloides, upon the then only 

 known species B. socotrensis of Pfeiffer, one of the most distinct 

 forms of the group. Achatmelloides (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 LXVIIL). 

 There is a third group of Bulimini peculiar to Socotra, c, charac- 

 terized 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 LXVIIL, 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 pre"^nt. 



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 iu 

 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. zehrinus we have another modification 

 of the same spiral bands seen in A. socotrensis, but in a still more 

 solid glossy shell. In A. balfoiiri, 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 irre- 

 gularly arranged, but still oblique to the costulation ; while in A. 

 ffollonsirensis, the most ornamented of ail, 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 iu this 

 paper, I think it due (as a member of the Committee for the explora- 

 tion of the island) to express to Prof. Bayley Balfour how much we are 



^ For Part I. vide supra p. 251 et seqq. 



Proc. Zool. Soc— 1881, No. LII. 52 



