344 MASON : VARIATION IN TflE SHELLS OF THE MOLLUSCA. 



in the drawer which completely reproduce the markings of the 

 Pedens to which they are attached. Other causes of variation 

 and distortion are the mixture of fresh and salt water, and the 

 chemical action of the water in which they live. This frequently 

 produces decollated forms. Parasites are not much responsible 

 among the moUusca for monstrosities, but, as Canon Norman 

 showed last year, an annelid is responsible for deforming Buc- 

 cinum undatum, whilst numberless parasites may be harboured 

 by some species, a certain stage in the development of the sheep 

 fluke by Limncea tmncatiila, without leaving any external mark. 

 The mechanical effects of the erosions caused by alg^, sponges, 

 and boring mollusca are easily distinguishable, and do not pro- 

 duce marked varieties. To take an instance of the way in 

 which a species may vary, nothing can be better than our 

 common Ptirpura lapillus. It may vary in size — be a giant, 

 medium-sized, or dwarf, have the shell thick or thin, the spire 

 elongated or shortened^ be dextral or sinistral, have ihe lower 

 whorls hollowed, compressed, or ventricose, the mouth narrowed 

 or expanded, the teeth of the peristome prominent or obliter- 

 ated ; the surface, quite smooth, be marked by strong spiral 

 ridges or plates alone, by longitudinal ridges alone, have both 

 longitudinal and spiral striae producing cancellse, or be covered 

 by beautiful foliaceous plates. If specimens be examined taken 

 from the crag, it will be found that in the seas of that epoch the 

 sculpture was even more prominent than in those procured from 

 our own seas. In colour, again, it may be absolutely M'hite, 

 both inside and outside, or uniformly coloured on each surface 

 in various tints of yellow, brown, and black, or be adorned in 

 different ways by variously coloured bands. The suture may 

 also be almost filled up or obliterated in some species, or be 

 channelled, or the whorls carinated. 



Time has not permitted me to do more than glance at 

 the different causes of variation and at the results produced, 

 and many more will, without doubt, at once occur to you. 



J.C., vii., Jan. 1894. 



