376 



PURPURA. 



tive operations by any ties of kindred ; for the shell of the Whelk 

 itself is not unfrequently found perforated, just as if by one of 

 its own species. 



990. To this family also belongs the Cassis, or Helmut-shell, 

 which is one of the largest of the whole Class. Most of the 

 species are inhabitants of tropical shores ; but a few are found 

 on the coast of the Mediterranean. They live at some distance 



from the shore, on 

 the sand, into which 

 they occasionally 

 burrow, so as to 

 hide themselves. 

 The shells of the 

 Cassis rufa and 

 other species are 

 beautifully sculp- 

 tured by Italian 



FIG. 659. CASSIS TVBERO8A. 



artists, in imitation 



of antique cameos; 



the different layers of colouring matter which they contain, strong- 

 ly resembling in hue those of the onyx and other precious stones 

 formerly used for this purpose. Of these, a great variety of or- 

 naments are made ; and of late years a considerable trade has 

 been carried on in them on the Continent. Numerous other 

 genera are associated in this group ; but it will be sufficient here 

 to notice the Purpura, a shell of comparatively small size, but 

 which is very abundant in some situations on our own coast and 

 elsewhere. The total number of known species is nearly one 

 hundred and fifty ; the largest among them are inhabitants of 

 tropical seas. It has been supposed that the Roman purple dye 

 was obtained from animals of this genus, and a small quantity of 

 colouring matter may be found in the Purpura lapillus of the 

 British coasts. It appears, however, that it was from species of 

 Murex ( 991) that the ancients procured their celebrated dye. 

 All the Mollusks of this group, so far as is at present known, 

 have the habit of depositing their eggs in egg-cases, or nida* 



