HALF AN HOUR WITH SHELL-FISH. 199 



pJiinus is a common species, and may be iden- 

 tified by its reddish zig-zag lines, Fig. 106. The 

 foot of this mollusc is very peculiar, moving one half 

 at a time. This feature, however, can only be 

 observed when it is placed in an aquarium. In 

 Trochus cinereus the top or apex of the shell is 

 usually worn down very much, as if it had been 

 dissolved away. It has markings very much like 

 those of the previously mentioned species, but may 

 be identified by its smaller size and eroded top. 

 Trochus majus is noticeable for its large umbilicus 

 the perforation to be seen at the base of the shell, 

 in the centre. The outside of this species has each 

 whorl sharply ridged. All the " Top-shells " have 

 opercula, or little doors, by means of which they can 

 close the apertures or mouths of their shells. This 

 is a common feature among the herbivorous univalves. 

 The round mouths of all the " Top-shells " indicate 

 that they belong to this class. 



In the " Pheasant- shell " (Pliasianella) the foot is 

 actually divided into two halves, so that it can pro- 

 gress one half at a time. So far, therefore, it is 

 allied to the " Top-shells." It is a very small 

 mollusc, but a most elegant shell, delicately mottled 

 with red and yellow zig-zag lines or blotches, and 

 having a porcelain appearance, which renders its 

 identification easy. Its entire length rarely exceeds 

 a quarter of an inch. Of the true periwinkles 

 (Littorina littorea) we have two varieties; one a 

 large, dark, slate-coloured individual, and the 



