72 Shell Life 



also occurs off the coast of Antrim, Ireland. It is 

 very active, and likes to climb and hang suspended 

 by its thread. 



The Modiolarias, though formerly regarded as 

 Crenellas, — which we retain as a popular name, — 

 differ from these in several small but sufficiently 

 important points. The mantle forms a distinct 

 incurrent tube in front and an excurrent one behind. 

 The foot is strap-shaped, and may be extended until 

 it is a mere thread two or three times the length of 

 the shell. The shell is rhomb-shaped, but instead of 

 the ribs on the surface extending equally all over the 

 shell they are in two series, which leave the centre of 

 the valves plain. The hinge-plate is notched as in 

 Crenella, but as a rule the central or cardinal tooth 

 is wanting. There are four British species. The 



Marbled Crenella (M. maronorata) though 



somewdiat angular is roughly oval in shape, 

 thin and glossy, pale green marbled with 

 red- brown. Its b^^ssus is almost trans- 

 parent, but tolerably strong. It lives 

 among the root-like suckers of the larger seaweeds, 

 in the crevices of rocks, the interior of empty shells, 

 and very commonly embedded in the tough coat of 

 the simple ascidians. An average specimen of 

 Ascidia mentula may yield as many as twenty of 

 these moUusks, which do not appear to cause any 

 inconvenience to the host. It occurs on all our 

 coasts, but must be sought below the lowest tide-mark, 

 in what are known as the Laminarian and Coralline 

 zones. If not satisfied with its present quarters, it 

 displays considerable activity in seeking others, 

 stretching its foot to its greatest extent, and having 



