b INTRODUCTION. 



A true shell is composed of one or more calcareous pieces, 

 commonly called valves, each piece formed by a series of 

 layers, applied obliquely upon each other, in such a manner 

 that each new layer begins within, and terminates a little in 

 advance of the one before it, 



STRUCTURE AND GROWTH. 



We shall now endeavour to describe the manner in which 

 the growth of each separate valve, or each regularly formed 

 shell, proceeds from the nucleus. 



Before the young animal has left the egg, if it be an 

 oviparous species, or the body of the parent if viviparous, the 

 nucleus of the shell is generally formed, and specimens are 

 sometimes preserved in which the young shell is seen within 

 the egg, as in the cut, fig. 1, 2 ; or adhering to the inner sur- 

 face of the full-grown shell by the dried mucus of the animal, 

 as seen in fig. 3. 



1. Egg of aBulinus. 2. The same broken, shewing the 

 young shell. 3. The young of a Paludina, as seen in 

 the aperture of the shell. 



In both cases, the nucleus is generally of a more horny and 

 transparent composition than the parts subsequently produced. 

 As soon as the animal is hatched, or, in other words, leaves 

 the egg or body of the parent, of course it begins to increase 

 in size, and to require a corresponding enlargement in the 



