INTRODUCTION. xxiii 



matter ; in all cases their form resembles more or less 

 that of a cone. 



According to Dr. Gray, "the shells of Mollusca 

 appear to be coeval with the first formation of the 

 animal ; they may be observed covering the embryo on 

 its first development in the egg, even before it has 

 acquired its proper shape or any of its internal 

 organs." * 



This first formed portion of the shell is called the 

 nucleus ; it constitutes the apex of all shells whether 

 bivalve or univalve, and usually remains attached to 

 them during all the stages of their growth. 



After the animal has emerged from the egg, it 

 gradually enlarges its shell as its body increases in 

 size, by adding, from time to time, fresh deposits 

 formed by the secretion of the mantle ; the place 

 where each successive deposit has been attached to the 

 preceding one is indicated by a line which is called the 

 line of growth. In most cases the shell consists of two 

 layers or coats : the outer one is membranaceous or 

 horny, and is called the epidermis ; the inner one is 

 calcareous, and constitutes the shelly part. 



The structure of univalve shells is more uniform than 

 that of bivalves, and usually contains less animal 

 matter. In ' British Conchology,' vol. iii. p. 202, 

 Gwyn Jeffreys says, "A univalve shell consists of 

 three layers of cellular plates, each of the upper two 

 layers lying unconformably on the one immediately 

 below it, and every plate being composed of a single 

 series of elongated prismatic cells which cohere length- 

 wise." And in the Introduction (p. 47) to the same 



* ' Philosophical Transactions ' for 1 833. 



