THE SHELL. 121 



d. The lines of growth are transverse ridges parallel 



to the mouth, and indicating successive positions 

 of the mouth during the growth of the shell. 



e. The umbilicus is an opening on the under surface 



of the shell leading into the hollow axis or 

 columella. It is sometimes closed. 



Glip away one half of the shell with bone forceps, and 

 grind the cut edges flat on a file or stone. 



f. The columella is the axial portion of the shell 



round which it is coiled : it is formed by the inner 

 wall of the several turns, each of which overlaps 

 rather more than half of the preceding one. 



g. The structure of the shell. The shell, like that of 



the mussel, consists of three layers, of which the 

 outer and middle ones are formed by the collar, 

 and consequently can only increase by additions 

 round the mouth of the shell. The inner layer 

 is formed by the integument covering the vis- 

 ceral mass, 

 i. The periostracum or outer layer is horny and 

 uncalcified. To it the colour of the shell is due. 

 It is usually rubbed off the uppermost turns 

 of the shell, leaving the middle layer exposed. 



ii. The middle layer is much the thickest of the 

 three. It is densely calcified, and has an 

 opaque porcellanous appearance. 



iii. The inner or nacreous layer has a glistening, 

 pearly appearance, and a very smooth surface. 

 It is formed by the superposition of a number 

 of delicate densely calcified laminae. 



2. The visceral hump is the part of the snail which is per- 

 manently lodged within the shell. It is the elon- 

 gated and spirally coiled dorsal part of the animal, 

 and contains the greater part of the viscera. It is 

 covered by a very thin layer of integument which 

 secretes the nacreous layer of the shell, and which 



