238 LIFE BY THE SEASHORE. 



is seen to bear two tentacles, which have two almost sessile 

 eyes at their bases. There are no lobes nor cirri such as 

 occur in Trochus. The foot is rounded at both ends, and 

 has a very distinct central groove. Make out these points 

 on a living common periwinkle, and then make a special 

 journey to the rocks to collect the different forms in the 

 living condition. Begin at high-tide mark and collect speci- 

 mens down to low-tide mark. Then, either indoors or at 

 the rocks, sort your specimens carefully. Pick out first the 

 common periwinkle (Littorina littorea), which is known by 

 sight to most people, and is almost always easy to recognise. 

 The shell is usually black, sometimes brown or red ; in the 

 young the surface is ridged, but the adult shell is often 

 nearly smooth. Having put aside all the specimens which 

 are obviously the edible kind, take out from the remainder 

 those with distinctly flattened spire, in which the coiled 

 part of the shell seems to be sunk into the last whorl. 

 This is L. obtusata. The shell has a peculiarly smooth 

 surface, and is very variable in colour, being usually shades 

 of yellow and brown. It lives chiefly among bladder wrack 

 (Fucus). Among the remaining specimens you will find a 

 number of yellowish colour, often banded, which, except 

 for their colour, present much general resemblance to the 

 common periwinkle. From it they differ especially in the 

 greater roundness of the whorls, and in the breadth of the 

 outer lip of the mouth, at the point where it joins the 

 columella, or pillar, which forms the central axis of the shell. 

 The result of this broadening of the outer lip is to give the 

 aperture the appearance of being partially filled up. This 

 form is L. rttdis, a species which lives near high-tide mark, 

 and is very variable, giving rise to several more or less 

 distinct varieties. The most distinct of these is the form 

 called L. patula, which has an ear-shaped shell with a 

 somewhat oblique spire. There are other species or varieties, 

 such as L. neritoides, a small form living above high-water 

 mark; but if those mentioned above are distinguished the 

 observer will do well. 



The special characters of the forms named may be briefly 

 described. The common periwinkle, Littorina littorea, is 

 denned by the combination of the following special charac- 

 ters : the surface of the shell, especially in the young stage, 



