234 



POPULAR CONCHOLOGY. 



towards the back of the shell. Some of the 

 genera have plaits or marks of various kinds on 

 the inner lip; but the form of the shells is 

 different from that of the Columellidce family 

 (which has the lip also plaited, but more dis- 

 tinctly), being much more globular. 



The Purpuridce derive their name from the 

 circumstance of some of the genera, especially 

 the Purpura, possessing a colouring matter, 

 which was made use of by the Romans to pro- 

 duce their beautiful purple dye ; their use has, 

 however, now been discontinued, the cochineal 

 insect having the same effect, and being more 

 readily obtained. There are thirteen genera 

 in this family, and all seem to possess an oper- 

 culum. 



99. Cassidaria. — Shell rather oblong ; spire 

 not much elevated; mouth 

 long, and rather narrow, ter- 

 minating in a canal, which is 

 recurved, but does not touch 

 the back ; left lip often rough 

 or wrinkled, and detached from 

 the pillar ; right lip thickened ; 

 exterior of shell transversely 



