36 MOLLUSCA.: 
benetits of the Mollusca are to be estimated: they 
are useful also in the arts. The pearl, that splendid 
auxiliary to costume in all ages, glittering on the 
tiara of the Assyrian monarchs and on the diadem of 
our own sovereign, is a production of this class of 
animals, This, it is true, has no merit beyond 
its beauty, but the substance called mother-of- 
CAMEO CUL IN A CASSIS. 
pearl is, as is well known, much used in the arts. 
{t consists of the interior layers of many species 
of shells, principally Bivalves; but some among 
the Univalves have an interior brilliantly nacreous. 
The Top-shells (Trochus), several species of which 
