224 BRITISH MOLLUSKS. 



TJnio Roissyi, Michaud (1831), Comp. de Drap. p. 112. pi. xvi. f. 27, 28. 

 TJnio margaritifer, Rossmassler (1835), Icon. vol. i. p. 120. pi. iv. 

 Margaritana margaritifer a, Lea (1838), Trans. Amer. Phil. Soc. vol. vi. 



p. 135. 

 TJnio brunnea, Bonhomme (1840), Mem. Soc. Aveyr. vol. ii. p. 430. 

 Alasmodon elongatus, Thompson (1840), Ann. Nat. Hist. vol. vi. p. 200. 

 TJnio tristis, Morelet (1845), Moll, du Port. p. 107. pi. xiii. f. 2. 

 Margaron (Margaritana) margaritifer and crassus, Lea (1852), Synops. 



Naiad, p. 39, 43. 

 TJnio (Margaritana) margaritifer, Moquin-Tandon (1855) Hist. Moll. 



vol. ii. p. 566. pi. xlvii. 

 TJnio (Lymnium) sinuatus, crassus, and ater, Moquin-Tandon (1855), Hist. 



Moll. vol. ii. p. 567 and 570, pi. xlviii. f. 1 to 3, and pi. xlix. f. 3 to 6. 

 Hah. Throughout Europe. (In rapids and mountain streams.) 



Mankind lias from a very early period delighted in adorning 

 itself with bits of iridescent shells. There is a peculiar charm in 

 the rainbow colours refracted by the sun's light, from their mi- 

 nutely undulated surface. The South Sea Islander displays the beau- 

 tiful phenomena of the spectrum in a string of Avicula fragments. 

 More civilized races select for their adornment the nacre de- 

 posited in superfluous globules, either on the shell's lining, or 

 within the fleshy parts of the animal. These abnormal secretions, 

 the produce of a surcharged or distempered gland, are the lovely 

 pearls which grace the necklace and the diadem. All nacreous 

 bivalves, and even bivalves like the giant clam {Tridacna), whose 

 shell is as opake as marble, produce pearls, but the marble pearl is 

 scarce, and when found not much valued. It is the iridescent pearl, 

 especially the pearl that is tinged with rose or salmon colour, apart 

 from its iridescence, which is the most esteemed ; and of our four 

 British Naiads, TJnio margaritifer, with its blush-tinted interior, is 

 the pearl-bearer par excellence. TJ. tumidus and pictorum are in- 

 habitants of gently flowing and stagnant water ; TJ. margaritifer 

 inhabits the mountain stream, where its pearl-secreting functions 

 are more stimulated by the turbulence of the waters. Pearls of 

 great beauty have been procured from specimens inhabiting the 

 Conway, North Wales. One of especial purity, presented by Sh- 

 it. Wynne to the Queen of Charles II., is now in the Crown of 

 Queen Victoria. 



TJ. margaritifer is diffused over the whole of Europe, appearing 

 in the South in a robust form, known as TJ. crassus. In Britain it 

 appears in all three kingdoms, chiefly in the mountainous parts, 

 not visited by TJ. tumidus or pictorum. 



