3\ut-shells ayid Mussels 73 



taken hold by its tip, pulling itself rapidly along by 

 contracting its foot again. Having secured a corner 

 to its taste, it spins its byssus and becomes fixed for 

 the time. The Ribbed Crenella (il/. costulata) is a 

 much smaller and brighter species, the shell 

 proportionately narrower, cream coloured marked 

 with purple-brown streaks, which are often zigzag, 

 and which show through the pale green epidermis. 

 Occasionally it attains the length of half an inch. 

 It is local on the coasts of Devon, Cornwall, Wales, 

 Donegal, and the Island of Herm, where it may be 

 found spun up under stones, or on small seaweeds, in 

 the lowest range of rock-pools, often with shell 

 fragments and gravel mixed up with its byssus. 

 The Green Crenella (71/. discors) is slightly longer 

 and much broader than the last, yellowish brown, 

 with a rather thick green epidermis ; less 

 angular than the others. The animal sews ^^^/^ 

 together branches of the common coralline %Jg^^ 

 and other small seaweeds that it may dwell Green 

 in safety within the nest. It is very ^[ntTrlor 

 generally distributed, and may be found 

 abundantly in rock-pools at half-tide and below. The 

 Black Crenella (IT. nigra) is so-called because in 

 aged specimens the epidermis assumes 

 a black tint ; more frequently it is 

 something between a purplish brown 

 and dark olive when looked at 

 Black creTIua closcly. Tlic colour and the sculp- 



ture at once distinguish it from the 

 others. The two sets of ribs from the beaks are 

 crossed by a third and coarser transverse set, which 

 form a network, often with minute knobs at the 



