THE MOLLUSCA 205 



by the inexperienced, is Fissurella grceca (or the " Keyhole 

 Limpet"). 



This resembles the limpet in general outline, although 

 rather more parallel-sided, and there is an opening at 

 the apex of the shell for the entrance of water to the 

 gills. 



The Trochi are represented on our shores by eleven 

 species, and many varieties of these. 



The most familiar is the purple " Top Shell " (Trochus 

 zizyphinus). This is the most beautiful of the genus, being 

 fairly large, about an inch in height, sharply conical, and 

 beautifully ridged and striated with alternate red and 

 blue bands. It is very common at low water on rocky 

 shores, preferring grottoes and fissures at the base of high 

 rocks. 



Trochus magus is our largest representative of the genus. 

 It is of more squat form than the last-named, and subject 

 to much variation in colour. It is very abundant in some 

 localities, preferring bottom, near low-water mark, where 

 there is a mixture of gravel and small stones. 



The forms that are found in such localities are usually 

 the largest, and are elevated. Specimens that are dredged 

 in deeper water are of more "depressed form, usually of 

 smaller size, and of brighter colouring chocolate and white, 

 bright red and white, etc. 



The pretty little Trochus striatus, in general outline like 

 T. zizyphinus, but taller in proportion, is common on the 

 leaves of Zostera at low water. The little scarlet- tipped 

 Trochus exasperatus, so much prized by collectors, lives in 

 the same locality, but among the small, loose stones. 



Trochus lineatus abounds in the Channel Islands, and is 

 common on most rocky shores, living high up on rocks 

 nearly to high-tide level. In the Channel Islands, and on 

 the opposite coast of France, it is largely gathered, and sold 

 in the markets as the " Grey Winkle," 



