446 



THE OPERCULUM. 



concentric rings ; and is surrounded by a thickened callous 

 deposit, which is broadest on the outer side (Fig. 79, 6). 

 This scar appears gradually to approximate towards the 

 right side of the operculum, the part left free, as the scar 

 advances, becoming covered by the callous deposit. The 

 lines on the scar appear to be very constant in the vari- 

 ous species, but they are sometimes distorted, and form 

 several centres instead of one. They have no relation to 

 the rings of growth on the outer surface, but have gene- 

 rally a centre of their own, placed at some distance from the 

 left end of the operculum ; and they appear to be formed 

 by the successive additions made to the edge of the adductor 

 muscle, which is marked with lines resembling those on the 

 scar. The greater number of these opercula do not alter 

 their relative position in the mouth of the shell ; but a few, 

 like those of Fusus fornicatus, in which the end of the cone 

 is slightly curved, move during their lives perhaps to the 

 extent of a quarter or half a turn on the end of the muscle. 

 In some instances, as in the operculum of S trombus, where 

 the foot of the animal is very small, the apex or left ex- 

 tremity is obliquely elevated and free : as the rings of growth 

 are added, this extremity elongates, and acquires such a 



Fig. 80. 



resemblance in form to the claw of an animal, as to have 

 induced the ancients to call such opercula Elks' hoofs (Fig. 

 80). The scar of attachment is small in comparison with 



