xiv INTRODUCTION. [CH. 



the characters derived from the animal, the shell, and the 

 operculum, which all have a mutual relation to each 

 other, are of equal value and constancy;" and he ob- 

 served "how thoroughly they depend on each other, 

 and what excellent and permanent characters they afford, 

 both separately and in combination with each other^." 

 Dr. Gray seems now to be quite sensible of the value 

 of conchological characters, as nearly the whole of his 

 numerous genera of MoUusca have no other foundation 

 than the shell. I believe that the form and structure of 

 the shell will be invariably found to correspond with the 

 habits and wants of the animal which produces it. The 

 mechanical principles involved in the construction of 

 shells, and the adaptability of these habitations to the 

 uses of their builders, have been admirably explained by 

 the Rev. Canon Moseley, in an essay "On the Geometri- 

 cal Form of Turbinated and Discoid Shells," which was 

 published in the ^Philosophical Transactions^ for 1838, 

 and to which I would invite the attention of my readers. 

 The uniformity of the laws which are here so ably ex- 

 pounded, and the correlation which exists between the 

 mollusk and its shell, exhibit in no small degree the in- 

 finite wisdom of the Creator of all things, thus 



"dimly seen 



In these thy lowest works ; yet these declare 



Thy goodness beyond thought, and power divine ! " 



Cephalic Mollusks or Univalves. — The Mollusca may 

 be considered in a general point of view as divided into 

 Cephalic and Acephalous, and their shells into Univalves 

 and Bivalves. The former arrangement was proposed 

 by Lamarck, and the latter by his predecessor, Linne, 

 who also added a third division — viz. that of Multi valves, 

 which would almost be warranted by the anomalous 

 * Ann. & Mag. Nat. Hist. Sept. 1855, p. 425. 



