242 



NATURAL HISTORY. 



Genus Unio,* the "River Mussel." The shell is rather stout; the hinge is composed of inter- 

 locking erect teeth on. the anterior side, and elongated marginal teeth, which are sometimes obsolete, 

 on the posterior. 



The Pearl- bearing Mussel ( Unio margaritiferd) afforded the once famous British pearls. It is found 

 in the mountain streams of Britain, Lapland, and Canada, and is used for bait in the Aberdeen Cod 

 fishery. The Scottish pearl fishery continued till the end of last century, especially in the River 



Tay, where the Mussels were col- 

 lected by the peasantry before 

 harvest time. The pearls were 

 usually found in. old and deformed 

 specimens. Round pearls, about 

 the size of a pea, perfect in every 

 respect, were worth 3 or <4. An 

 account of the Irish pearl fishery 

 was given by Sir R. Redding, in 

 the Philosophical Transactions, 

 1693. The Mussels were found 

 set up in the sand of the river-bed, 

 with their open side turned from 

 the torrent ; about one in a hun- 

 dred might contain a pearl, and 

 one pearl in a hundred might be 



vxio PICTORUM. 



tolerably clear. 



Hyria is the shell which the 



Chinese employ to produce artificial pearls, by the introduction of shot, <fcc., between the mantle of 

 the animal and its shell. A Hyria in the British Museum has a number of little josses made 

 of bell-metal, now completely coated with pearl, in its interior. The river Mussels are found in 

 the ponds and streams of all parts of the world. In Europe the species are few, though specimens 

 are abundant ; in North America, both species and individuals abound. All the remarkable generic 

 forms are peculiar to South America and Africa. 



Genus Anodon. The shell is thin and toothless. This is the largest of European fresh-water 

 molluscs. It has a very inflated shell ; the valves, although toothless, are united by a strong external 

 ligament ; the foot of the animal is very large, and of an orange-yellow colour. The lakes, 

 canals, ponds, and gently-flowing rivers through 

 Europe, are all tenanted by Anodonta. They 

 are very abundant in North and South America. 

 Several hundred species have been described, 

 but they are, in all probability, capable of being 

 reduced to half a dozen, so great are the varia- 

 tions which these shells present. 



Nearly all the great rivers of the world 



have some characteristic form of the genus Unio. Thus Castalia is peculiar to the rivers of South 

 America, especially the Amazon. Iridina occurs in the rivers of Africa, as the Nile and Senegal. 

 Mycetopus, a Solen-like form of Unio, is found in South America only. 



Two genera (^Etherid), from the River Nile (first noticed by the African traveller Bruce as a "fresh- 

 water Oyster "), and Miilleria, from New Granada, are fixed and ii'regular when adult, and have been 

 placed with the CJiamas and Oysters by the admirers of artificial systems ; fortunately, however, 

 M. D'Orbigny has ascertained that the Miilleria, which is fixed and mono-myary when adult, is 

 locomotive and di-myary when young, like any other Unio. (S. P. Woodward). 



Mollusca inhabiting fresh water are especially exposed to corrosive action, either from carbonic 

 acid in solution or dilute sulphuric acid from the decomposition of iron pyrites. But the action is 

 especially manifested in those stagnant waters where the first probe of the collecting-rod disengages 



* Latin, iinio, a pearl. 



AXOnOXTA EXSIFORMIS. 



