246 COMMON FRESHWATER SHELLS. 



While we are on this subject, the remarks of Tacitus on 

 British pearls might be noticed. He says : — " The ocean also 

 yields pearls, but they are cloudy and discoloured. Some believe 

 that the gatherers are deficient in skill, because in the Red 

 Sea they are torn alive and breathing from the rocks ; while in 

 Britain they are merely collected as they are cast ashore ; but I 

 should rather suppose that the quality of the pearls is inferior, than 

 that we are deficient in avarice." 



It is well known that pearls are produced by some foreign 

 matter becoming lodged in the shell in such a way as to annoy the 

 occupant, and cause it to coat that matter over with a nacreous 

 substance in order to make it smooth and less painful. The 

 Chinese take advantage of this practice, and introduce into the 

 shells of mussels ' josses ' and other articles in order to obtain a 

 coating of pearl. 



Sand was generally said to be the nucleus of pearls, " but this 

 is simply a conjecture which has gradually become regarded as a 

 fact. It is quite the exception for sand to be the nucleus ; as a 

 general rule, it is some organic substance." The most generally 

 prevalent nuclei appear to be the bodies or eggs of minute 

 internal parasites. The prevailing colour of the pearls of Unio 

 margaritifer is white ; but green, brown, black, and flesh-coloured 

 or pink pearls are occasionally found, those of the last-mentioned 

 colour, when large and well shaped, being of considerable value. 



The shell of this species of mussel is very thick and heavy. 

 It is about five and a-half inches long, and is covered with a 

 strong, dark-coloured, or in some cases black, epidermis. The 

 interior of the shell is very thickly coated with mother-of-pearl. 



Extinct Plants. — The number of species of plants which 

 have become extinct is very large, and yet generic groups rarely 

 die out. Comparative researches show that much the greater pro- 

 portion of plants whose remains have been preserved in a fossil 

 condition from earlier geologic periods belong to the genera which 

 are represented by plants now living, although many of these 

 existing plants differ specifically from the earlier ones. From this 

 it seems that new types are outgrowing the old ones constantly, 

 and take their place in the general scheme of life. — -Journal of 

 Horticulture, 



