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 



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 



