372 POPULAR BRITISH CONCHOLOGY. 



times free. It is however generally fond of retirement ; if 

 it can find a rock soft enough to be easily pierced^ it does 

 so ; and if it can find a crevice or hole in harder rock, it 

 creeps into it, and adapts its growth to the shape of the 

 cell. The British species are S. arctica, the shell of which 

 is generally spinose; and S. nigosa, which is rough and 

 irregular, but witliout the spines. 



From the very variable forms assumed by both the above 

 species, they have received many generic and specific names. 

 But these variations may be considered as the result of a very 

 enviable power possessed by the animal, namely, that of adapt- 

 ing itself to its position and circumstances. 



III. Petricola is a rather similar mollusc to the last, 

 but has the siphons much more divided, and the shell has 

 teeth on the hinge. The genus is represented in Great 

 Britain by P. lithojohaga. 



lY. Yenerupis. — Shell more Venus-\ike, and animal 

 very similar. Eepresented among British molluscs by F. 

 Irus (Plate lY. fig. 2). 



These two last appear to be much more nearly alHed to 

 the Venus tribe than to Gastrochcena. 



