94 ASHFORD : ON BANDED LAND SHELLS. 



We come now to the consideration of the second primitive 

 zone situated b^low the periphery. This I think was mukiple in 

 all the four species, but it is almost impossible to detemine the 

 number of the group when it existed before changes set in. 



These changes have been so numerous that it is very difficult 

 to suggest a mode of classification at all methodical, but this is 

 the less to be regretted as the undersides are less generally exposed 

 to view in museums. They are not however on that account less 

 worthy of attention. The modification of this multiple zone are 

 chiefly conspicuous (i) by the nonproduction of some or all of 

 its members; (2) by their multiplication; {3) by their confluence; 

 (4) by obliteration (5) by diffusion. The causes of the last two — 

 obliteration and diffusion — appear to be the same as those which 

 have prevailed on the upper side, but since the stride are usually 

 less developed below we are not surprised to find as the result 

 that the inferior zone shows more power of endurance than the 

 superior one, and sometimes exists entire when the latter has be- 

 come obliterated. The causes of absence, multiplication and con- 

 fluence must be looked for rather in modifications of the secretive 

 glands of the animal than in the outer structure of the shell. 

 Examples of these variations are so numerous in the case of the three 

 Helices that we need only remark upon the Bulvims. Instances 

 of a compound zone occasionally occur in the case of B. aciitus; 

 but as this species has reached a more advanced stage of change 

 than the Helices they are less frequent. It may be objected that 

 these exceptions may be accounted for by multiplication as in the 

 equally exceptional six and seven-banded individuals of H. nenio- 

 ralis, and that had the original ancestors of B. acutiis possessed 

 such a compound zone more frequent vestiges of it would be 

 found among the very numerous individuals which retain the 

 inferior band. This objection has great weight ; but does not 

 the fact (as I believe it to be,) that the exceptions referred to 

 occur only among individuals of the var. bizona — the section of 



