J INTRODUCTION. [CH. 



80 fathoms ; and not only were live shells which I pro- 

 cured from those depths as brightly coloured and marked 

 with as distinct patterns as shells of the same species [e.g. 

 of Trochus ziziphinus), taken at low- water mark, but 

 colourless or white varieties of such species were found 

 in the same spots. The Star-fishes lately got by Dr. 

 Wallich in the Arctic Sea from a depth of 1260 fathoms 

 still retain their former colours ; and, during the recent 

 expedition of Torell and other Swedish naturalists to 

 Spitzbergen, a portion of the sea-bottom was brought up 

 from a depth of 1400 fathoms, when, among other ani- 

 mals of different types, a Crustacean of bright colours is 

 said to have made its appearance. The extent to which 

 light penetrates into the abysses of the ocean, as well as 

 the mode of its transmission, does not seem to be known. 



Decollation, — Some univalve Mollusca, both terrestrial 

 and aquatic, the shells of which have a long and slowly 

 enlarging spire, desert the first or top whorls, and get 

 rid of them by a process called decollation or truncature. 

 The suture, or point of junction between this part of the 

 spire and the rest of the shell, is usually very slight ; 

 and the animal effects the process of decollation by 

 ])urying itself in the earth if a land- snail, or rubbing 

 its shell against a stone or other hard substance if a 

 freshwater or marine species, in order to disengage itself 

 from the empty and useless whorls. Before doing this, 

 however, it reconstructs the top of its spire by forming 

 a hemispherical plate of shelly matter between that part 

 of the shell Avhich is to be retained and the empty top. 

 Among land-snails Bulimus decollatus is a well-known 

 instance of this peculiarity, among freshwater snails 

 Limnaa glabra, and among the marine univalves Trun- 

 catella truncatula. 



Erosion. — The outer layers of the shells of aquatic 



