260 SHELLS AND SHELL-FISH. PART I. 



(245.) As the whole of these shell-fish are inhabit- 

 ants of fresh waters, and chiefly of rivers, so are they 

 most abundant in those countries whose lofty mountains 

 supply copious and never-failing supplies of their fa- 

 vourite element. The mighty rivers of the New World 

 are no less remarkable for their innumerable rami- 

 fications, than for the almost endless number of these 

 shell-fish which live in them. The American spe- 

 cies, in fact, comprise more than seven tenths of the 

 whole number yet made known ; five or six are alone 

 found in Europe ; about the same number occur 

 in Australia, a few more in Asia, and we have as yet 

 seen none from Africa. Why the immense rivers of 

 the East should be so destitute of Unionidce, is a ques- 

 tion we know not how to solve. One reason may pro- 

 bably be, that they have not been sufficiently examined. 

 Before the researches of Spix and Martius in Brazil, 

 scarcely five were known from South America, so that 

 nearly all that they found were new, and others will no 

 doubt be soon brought to light. It is difficult, indeed, 

 to assign any reason why the tropical rivers of the New 

 World should not produce these shell- fish at least as 

 plentifully as the more temperate waters, such as the 

 Ohio and its tributary streams. The Unionida almost 

 always have their bosses more or less rubbed and in- 

 jured, occasioned by the friction of stones and other 

 substances carried onward by the stream. Although 

 generally of an olive brown colour, occasionally varied with 

 obscure rays on their outer surface, the interior of these 

 shells are often particularly brilliant, and their sub- 

 stance is always perlaceous. Hence they are now manu- 

 factured in America into pearl buttons and other orna- 

 ments. Real pearls, also, are occasionally found in some 

 of the species. We once took from a small specimen of 

 U. circulus, a beautiful little pearl, half the size of a small 

 pea, which is now in the possession of our friend Mrs. 

 Corrie of Birmingham. The My a rnargaritifera Linn., 

 still found in some of our own rivers, produced those 

 pearls for which Britain was so famous in the time of the 



