III.] INTRODUCTION. xlvii 



the apex or nucleus of each valve being analogous to the 

 same part in a spiral shell ; but in bivalves the deposit 

 of shelly layers is formed on both sides, in consequence 

 of the mantle or organ of secretion being double. Every 

 part of the shell, whether univalve or bivalve, enlarges by 

 growth in the same relative proportion. 



Nucleus. — This part of the shell furnishes the concho- 

 logist with an important character of distinction. Odo- 

 stomia, Nassa, Cerithiopsis, lanthina, and many other 

 genera of univalve shells have the apex of their spire 

 differently formed. In the adult Cyprcea (or Cowry), 

 the whole of the spire, including the apex or nucleus, 

 is covered and concealed by an extraneous deposit of 

 shelly matter; and the same process, although to a 

 partial extent only, takes place with regard to the genus 

 Marginella. In many spiral shells the upper whorls 

 (which originally formed the nucleus) are deserted by 

 the animal, in consequence of the volume of its body 

 having increased so rapidly that these whorls were too 

 small to receive any part of it, and they therefore became 

 unnecessary. 



Growth. — Owing to the difficulty which exists in keep- 

 ing and observing Mollusks in a state of confinement for 

 any length of time, and especially those which live in the 

 open sea, very little is known as to the mode and rate 

 of their increase. Some interesting experiments on the 

 growth of land-shells were, however, made by Mr. E. J. 

 Lowe and communicated to the Royal Society in 1854 *. 

 The result of his observations is as follows : — 1st, The 

 shells of Helicidce increase but little for a considerable 

 period, never arriving at maturity before the animal has 

 once become dormant (or hibernated). 2nd, Shells do 

 not grow whilst the animal remains dormant. 3rd, The 



* Proceedings, vol. vii. p, 8. 



