18 



MALACOZOA. 



distinguished into central and lateral. The surface 

 may be convex in various degrees, concentrically striate, 

 laminate, or rugose, or radiated from the umbones with 

 striae, ridges, grooves, ribs, or spines. In the natural 

 position, the hinge is uppermost on the back ; that end 

 of the shell to which the ligament is nearest is above, 

 and is called the posterior end ; the other or lower, 

 toward which is the head of the animal, being the an- 

 terior end ; the thin edges of the valves are their ventral 

 margins. On the inner surface of the valves are seen 

 the impressions made by the muscles, and that left by 

 the mantle. 



Some Mollusca live on land, others in fresh-water, 

 but by far the greater number in the sea ; some residing 

 along the shore, others in deep water, and some floating 

 free. They are more abundant in warm than in cold 

 climates ; larger and more brightly coloured the"greater 

 the heat and light to which they are subjected. This 

 influence is perceptible within the range of even our own 

 island ; for in the South of England the same species 

 of Snail, for example, are larger and more beautiful than 

 in the North of Scotland. 



The district of which the natural productions are de- 

 scribed in this work, although hitherto represented as 

 extremely unproductive, contains a fair proportion of 

 the British Mollusca ; but on this subject some remarks 

 will find a more appropriate place at the end than at 

 the beginning of the description of the species. 



It may be observed, that the Malacozoa have been 

 variously arranged by authors, and that one might dis- 

 play a great deal of useless ingenuity in exposing and 

 criticising the proposed systems . According to the method 



