THE FIRST SHELL. I3 



those with circular apertures are usually vegetable 

 eaters. 



Just to the left of the caual is a little chink or hole 

 called the umbilicus, marked //. In some shells the 

 umbilicus is very large, and is the space left as the 

 turns of the shell grow in a spiral form, curving 

 around a central opening. Each turn of the shell is 

 called a zvhorl ; the last and largest one, b.w.^ is 

 the body ivhorl. The earlier and smaller w^horls make 

 up the spire^ sp., and the line of union between two 

 whorls is called a suture, which is marked s. in our 

 picture. Thus we come round again to the apex, a.^ 

 from which we started, and from which our little 

 creature started also ; for wdien it was hatched from 

 an ^gg, it had a minute shell of two or three whorls, 

 which remain, in part at least, as the whorls nearest 

 the apex. 



As time went on, the whorls increased in number, 

 the outer lip being constantly built up, and the aper- 

 ture thus going round and round the central axis of 

 the shell, and all the time growing larger and larger. 

 The outside of the shell is not perfectly smooth, but 

 is marked by about twenty little grooves and an equal 

 number of ridges, which follow the shell round and 

 round, from the apex to the edge of the outer lip, 

 where you see them very distinctly. We will call 

 these markings spiral lines, to distinguish them from 

 other markings on the shell. 



The growth of the shell is frequently interrupted 

 for a little time, and if you follow back from the edge 

 of the outer lip, you can see numerous little lines, 

 parallel with the present edge, and curved just as the 

 edge is now. These marks, w^iich run across the 

 spiral lines, are called lines of groivth, and show you 

 just where the lip ended when the shell was younger. 



