^Hjit-shelh and Mussels 7 i 



composed of " inother-o'-pearl." The mantle forms 

 only one siphon, — that for getting rid of waste, and 

 this is very short, — fresh supplies being taken in by 

 the open front. The foot is of peculiar construction, 

 consisting of a cylindrical stalk and a tongue-shaped 

 expansion which is used for crawling. Instead of 

 spinning an extensive byssus, these content them- 

 selves with a single thread by means of which they 

 suspend themselves. The genus is now restricted to 

 two species, though formerly those now forming the 

 genus Modiolaria were included in Crenella. 



The Glossy Crenella {G. rhombea) has a solid- 

 looking, glossy white shell, marked by delicate 

 radiating ribs, and covered by a slight pale yellow 

 epidermis. The broad hinge-plate is notched or 

 crenulated as already noted, and bears a wedge- 

 shaped tooth with notched edges. The interior edges 

 are also crenulated, so that the valves securely 

 interlock all round. It is less than one-quarter of an 

 inch in length, and occurs on rocky coasts in the 

 south-west of England and round the Channel Islands 

 from low water to about 20 fathoms. The Cross- 

 cut Crenella (0. decussata) is more oval than e?^ 

 the last, of the dimensions shown in the fipfure. ^^ 



o Cross- 



It is also glossy, but the number of ribs is cut 



smaller, and they are crossed by fine hair- ^^^"^"^ 

 like lines. At the points of contact these lines are 

 slightly raised into minute rounded points. The epi- 

 dermis is thicker and of a darker hue, so that the 

 general appearance of the shell's colour is pale olive. 

 It affects gravelly sand on our northern coasts, from 

 Yorkshire and Northumberland to the Slietlands, 

 and ranging from 3 to 70 fathoms in depth; it 



