b INTRODUCTION. 



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

 commonly called valvesj 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. 



The Cirripedes, however, although their testaceous pieces 

 will correspond with this definition, are otherwise so con- 

 nected with the Crustacea, that they do not enter into the 

 study of Conchology. 



STRUCTUEE AND GEOWTH. 



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. Eg'g' of a Bulinus. 2. The same broken, shewing; the 

 youn^ shell. 3. The young of a Paludina, as seen m 

 the aperture of the shell. 



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



