408 UNIO. ARCACE^E. 



and thousands of young ones may be seen in the winter, with a 

 microscope, dispersed among the gills, and opening and closing 

 their shells. The Unio resembles the Anodon in the structure of 

 the shell, and the conformation of the animal ; except that the 

 hinge is more complicated. There is a short plate in the left 

 valve, received into a cavity in the right; and behind this a 

 longer plate closing between two others of the opposite side. 

 These, like the Anodontes, inhabit fresh-water, preferring run- 

 ning streams. Several are natives of this country; but they 

 especially abound in the rivers and lakes of North America. 

 The abundance of their nacreous lining causes it sometimes to be 

 employed for the purposes to which mother-of-pearl is applied ; 

 and pearls are occasionally obtained from them ( 943). The 

 animal is of no value as food, from the insipidity of its taste. 

 Some genera of marine shells have been approximated to the 

 Unio ; but as their animals are not known, there can at present 

 be no certainty on this point. 



ORDER VI. ARCACE^E. 

 950. The Area is distinguished by its equivalve shell, and by 



FIG. 593. ARCA BARBATA AND ARCA N<XE. 



the long line formed by the hinge, which is studded with minute 

 teeth. The valves, which are covered with a velvety epidermis, 

 do not meet in the middle ; but a space is left for the passage of 

 a horny, apparently tendinous substance, which seems to replace 



