MOLLUSCS, OB SHELL-FISH. 239 



is marked by striae, the whorls are more or less flattened, 

 the outer lip of the aperture joins the last whorl at an 

 acute angle, and is more arched below than above. On the 

 last point special stress should be laid, as it is very character- 

 istic. The colour of the shell has been already described'; 

 as to the colour of the living creature, the fact that the 

 horns and tentacles are spotted and ringed with black is 

 especially noteworthy. 



It is much easier to distinguish between actual specimens 

 of L. littorea and the next species, L. rudis, than it is to say 

 wherein the difference actually consists. In the latter the 

 whorls are distinctly rounded, the outer lip joins the last 

 whorl at a right angle, and is more arched above than 

 below. This, which is an important difference from the 

 common periwinkle, may seem a very trivial matter, but it 

 has, in reality, considerable bearing on the life-history. The 

 common or edible periwinkle lays eggs on Fucus in little 

 jelly-like patches, a habit which is no doubt the primitive 

 one for the species. But such a habit is obviously im- 

 possible for forms like L. rudis and its varieties, for they 

 inhabit localities often not covered by every tide, and un- 

 suited to the growth of the tangles. It therefore retains its 

 eggs within the body until the .young develop, and they are 

 subsequently born already furnished with shells. There 

 can be little doubt, I think, that the shape of the shell- 

 mouth bears a direct relation to this viviparous habit it 

 allows room for the young to develop, and makes birth easy. 

 Practically, the viviparous habit is of some importance, 

 because it renders this species unfit for food, owing to the 

 grittiness imparted by the presence of the young during 

 several months of the year. The species never reaches the 

 size of the preceding. 



The form called L. patula is merely a variety of L. rudis, 

 but lives even further up on the shore. It is usually smaller, 

 has a thinner shell and a more stunted appearance, the 

 whorls, especially the last, are more expanded, and the 

 aperture of the shell is wide. As in L. rudis, the tentacles 

 of the living animal are usually marked with longitudinal 

 stripes, not with rings or spots, as in L. littorea. 



Typical examples of L. obtusata are so easy to recognise 

 that it seems unnecessary to describe their characters 



