20 TESTACEA. 



arrangement of natural history offered for the guidance of students. 

 Conchologists not having hitherto determined the precise arrangements 

 of the Testacea, and modifications being daily proposed, render me less 

 scrupulous regarding the methodical order of a subject which did not 

 form a part of my original plan. 



Besides the systematic arrangement of sheila, the notice of natu- 

 ralists has been directed to their constituent parts, whereby many beauti- 

 ful and interesting facts are exposed. These will be found especially 

 illustrated in a Treatise by an accomplished author, Dr Carpenter, in the 

 Cyclopaedia of Physiology and Anatomy. 



Avoiding the nicer distinctions, we shall consider the Testacea as 

 partitioned into two great divisions, the univalves and the bivalves : the 

 former, where the animal has only a single shell, of whatever form, for 

 its protection ; the latter, where it is guarded by two, or enclosed in an 

 impenetrable chest. 



During the life of the tenant and in earlier age, many of the shells are 

 vivid and beautiful ; but then- freshness and colour decay with its death. 



The fabric and appearance also alter very much with age. Some, 

 especially among the bivalves, gain great and irregular accessions ; while 

 most of the univalves have suffered abrasions, fractures, and other serious 

 injuries. 



Young shells continue to grow during a long time ; and as their 

 tenants obviously attain a very advanced age, the cessation of the in- 

 crease seems quite unknown. 



A regular traffic is carried on in shells in different parts of the world, 

 which is liable to the greatest fluctuation ; for shells, like some other 

 subjects, being prized according to their rarity, the discovery of some 

 previously unknown store produces an absolute revolution in the market. 

 The price given for some such rarities, like that for certain old coins and 

 medals, is little indicative of prudence, taste, or reason. 



. The finer shells certainly belong to the warmer climates. Many 

 are of more curious form, and more vivid colours, than those of the colder 

 countries : They also attain much larger size. 



A few of our own country, however, both terrestrial and marine, 



