IxXViii INTRODUCTION. [CH. 



smallest weight used by apothecaries, are among our 

 greatest treasures. As is not uncommon in such cases, 

 beauty of form or brilliancy of colour does not always 

 represent the same value as rarity and the consequent 

 difficulty of acquisition. As much as 8 has been given 

 for a single specimen of Panopcea Norvegica, which 

 would be considered by all but conchologists as a very 

 ugly and coarse shell. Some of our scarcer kinds of 

 Fusus also command good prices ; and it is said that the 

 magnificent specimen of Buccinum acuminatum, which 

 once belonged to Lord Kilcoursie and is now in our 

 National Museum, cost the Trustees (or rather the 

 country) no less than 12. Mr. Damon of Weymouth, 

 as well as Mr. Rich^ Mr. Sowerby, and Mr. Wright, all 

 of Great Russell Street, London, are the principal dealers 

 in this line ; and a priced list of British shells may be 

 had of the first-named enterprising person. 



CHAPTER V. 



DISTRIBUTION. 



EXTENT. ORIGIN OF BRITISH MOLLUSC A. FOSSILS. GULF- 

 STREAM. 



Extent. It has long been notorious that distinct 

 groups of Mollusca, as well as of other animals and of 

 plants, occupy more or less extensive areas of the earth's 

 surface. This distribution is generally more limited with 

 regard to terrestrial kinds than to those which have an 

 aquatic habitat. The temperature of the sea at certain 

 depths is constantly the same everywhere, and it does not 

 appear to be affected by that of the surface. One species 



