xxiv INTRODUCTION, 



work, he makes the following remarks respecting the 

 growth of shells : " Owing to the difficulty which exists 

 in keeping and observing molluscs in a state of con- 

 finement 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, how- 

 ever, made by Mr. E. J, Lowe, and communicated to 

 the Royal Society in 1854. The result of his observa- 

 tions is as follows: — ist. The shells of Helicidce in- 

 crease but little for a considerable period, never 

 arriving at maturity before the animal has once be- 

 come dormant (or hibernated). 2nd. Shells do not 

 grow whilst the animal remains dormant. 3rd. The 

 growth of shells is very rapid when it does take place. 

 4th. Most species bury themselves in the ground to 

 increase the dimensions of their shells. Helix pom atia 

 and many other shells retreat for that purpose in 

 summer, having their heads and the mouths of their 

 shells downwards (this position being reversed during 

 hibernation) ; H. rotimdata burrows into decayed 

 wood for the same purpose ; while Pupa umbilicata^ 

 Clausilia nigricans {C. rtigosa), and BulimiLS obsciirus 

 bury their heads only while the increase takes place." 



The colouring matter which imparts so many 

 beautiful and varied tints to the surface of shells, is 

 secreted by glands in the mantle, which are specially 

 designed for the purpose. When the secretion 

 happens to be defective or wanting, the shell is more 

 or less deficient in colour, and is called a white 

 variety. 



Opei'culum. — The aperture of many univalve shells 



