94 PROCKEDTNGS OF THE MALACOI.OGICAL SOCIEIY. 



which refers tlie difference to causes purely mechanical. A long spiral 

 shell which is accustomed to live and grow without support is likelv 

 lo become produced in the spire, pure!}' by the operation of its own 

 weight, while a shell which is to a certain extent supported, or at all 

 events which has not to bear the continual downward pull of its own 

 weight, would not exhibit this tendency to become produced. It 

 follows that shells which creep liabitually upon the surface of 

 perpendicular rocks will tend to be, as a rule, longer than shells of the 

 same species which creep on the level, or on a surface approaching 

 the level, where the effect of weight is not so pronounced. That this 

 actually occurs I have noticed in the cases mentioned above, particularlv 

 in that of livida, Menke, where specimens taken from boulders in the 

 Malajester Schlucht were appreciably smaller than specimens from 

 the perpendicular cliff's which rose a few score yards away. 



5. Derivation of the group. — One is tempted to indulge in speculations 

 as to where the origin should be looked for of this interesting group, 

 which, from the absence of a clausilium in some of its species, and its 

 instability in others, appears' to be a survival of a stage in the 

 development of Clausilia, whose nearest relatives must be sought for 

 in groups now extinct and represented only bj- fossil or sub-fossil forms. 

 It seems quite clear that the group, as it is now represented, originated 

 high up on the mountains,- and did not climb up into them from the 

 plain. One is led to this conclusion by observing that when the 

 conformation of a mountain admits of it, a species descends low ; where 

 it does not, it remains high up. Thus, A. livida, Menke, begins at 

 about 200 feet below the Schutzhiitte in the Malajester Tal, that is, 

 at about 5,000 feet, and continues up to the top of the Butschetsch 

 (8,230 feet). It comes no lower, because the great boulders and cliffs 

 cease in that Schlucht at about 5,000 feet, and it cannot live except 

 on these. On the other hand, the forms characteristic of the Propasta 

 and the \\\:(i'^^t\\Vix{Lisc]ikeana, Parr., and Fussiana, Bielz, var. insignis) 

 almost reach the level of the plain near Zernest (about 2,000 feet), 

 because the Schluchten in which they live spring almost from the 

 plain itself, and they have thus been able to descend the mountain to 

 the lower level. The most striking instance of this is plionhea, Possm.. 

 which occui's all over the Schuler (5,900 feet), the nearest mountain 

 to Brasso, but it is also found on the walls of Brasso itself (1,800 feet), 

 because (1) there occurs between the Schuler and Brasso a series of 

 cliff faces and rocks, never separated by a very distant interval, and 

 (2) the species seems to have accustomed itself, more than any other 

 of the group, to living on smaller rock-faces and smaller boulders. 



Questions relating to the validity and relationship of the various 

 species and varieties will be dealt with in a further paper. 



Boettger, Clausilienstudien, pp. 6, 24. 



Kimakowicz is bold enough to regard the Czukds and the Butschetsch Mountains 

 as the two original homes of the parent stock or stocks. 



