302 TAYLOR : ON THE VARIATION OF MOLLUSCA. 



Many of our land species would almost seem to be changing 

 the character of their markings. Nearly seven years ago Mr, 

 Ashford suggested to me the probability that Helix cantiana, 

 cartusiana, &c., were once banded species, and I am disposed 

 to agree with his suggestion. As is well-known many of our 

 species which are usually somewhat uniformly colored show at 

 times a light peripheral or keel line. I may instance H. Mspida, 

 rufescens and cantiana as sometimes showing this clearly. 

 Species which have not advanced so far in the process of 

 change give us the clue to the meaning of this : thus H. virgata 

 has often the upper and lower groups of bands coalesced, leaving 

 a keel line of the ground colour, this is then exactly the corres- 

 ponding variety, of say H. rufescens with the light keel line. 

 When rufescens or hispida is uniformly darkly colored, they have 

 their representative in the var. nigrescens oiH. virgata in which 

 the pigment is suffused over the whole shell. In H. cantiana the 

 bands seem to be in process of elimination, and now only give 

 signs of their existence near the mouth when the animal is at its 

 highest vigor, with the strongest development of its organs. 



In connection with this subject of banding Herr Dietz 

 has remarked that wet seasons prevent the formation of colored 

 bands in IT. hortensis, and as the result of his observations, says 

 that albine specimens are more common in wet years, and that 

 those specimens with coloured bands have the growth of the 

 last wet year not colored. 



Another curious phenomenon is the scalariform specimens 

 of the genus Planorbis which are often found in those bodies of 

 water choked up with vegetation, and according to Herr Clessin, 

 also on the margins of lakes amongst large stones. M. Van den 

 Broeck considers this spiral form as a modification consequent 

 upon and adaptive to the special and peculiar features of their 

 environment, as he has conclusively proved by experiment, that 

 these spirally coiled shells make their way more readily through 

 the dense vegetation, than those of normal shape which traverse 

 the thick masses of duckweed, &c., with great labour and 

 difificulty. 



J.C, v., April, 1888. 



