THE MOLLUSCA 207 



to qualify for a terrestrial life. It is found of all colours, 

 red, white, yellow, etc., and banded with these colours. 



Natica catena and Natica alderi (the " Sea Snails "). 

 These beautiful, large, white and pink molluscs live on most 

 of our shores where there is soft shell sand, from low -tide 

 level to some fathoms deep. 



Curious biscuitlike cakes of sand, very firm and yet 

 elastic, often puzzle the sea-side visitor. These are the 

 egg cases of Natica, embedded and protected by this 

 agglutination of sand grains. 



Purpura lapillus (the " Dog Whelk ") is familiar to every 

 sea-side visitor. It lives near high-tide level, on nearly 

 dry rocks, and does not extend to the lower zone. It lives 

 chiefly on the limpet, drilling smooth, round holes at one 

 side of the shell near the apex, and then, inserting its pro- 

 boscislike mouth, feeds upon the unlucky, living limpet. 

 If a dog whelk is seen perched upon a limpet, and it is 

 carefully lifted off, the process of drilling will be seen either 

 in operation or just completed. Dead limpet shells strewn 

 around show by the holes the extent of the dog whelk's 

 ravages. It also feeds upon the common little trochus 

 (T. umbilicatus), but does not trouble to drill the shell 

 of this, simply turning the trochus over and sucking him 

 out by the opening of the shell. 



The egg cases of Purpura lapillus are curious and con- 

 spicuous objects in rock crevices and attached to stones 

 near high- water line. Fig. 88 shows a colony of these 

 egg cases. 



Murex erinaceus (the " Rock Winkle ") is another mollusc 

 borer, and is the plague of the oyster grower. This lives 

 at a lower zone than the dog whelk, chiefly on rocky, but 

 also on stony and gravelly, ground.. 



If the flat side of a few oysters is examined, on the 

 smooth part near the hinge smooth, round holes, the 

 attempts of this mollusc to get to the inside, may often be 



