408 UNIO. ARCACE.E. 



microscope, dispersed among the gills, and opening and closing 

 their shells. Dr. Lea has calculated that as many as 600,000 

 young are contained in the outer gills of a single female. 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 clos- 

 ing between two others of the opposite side. These, like the 

 Anodontes, inhabit fresh water, preferring running 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 ( 1027). The animal is of no 

 value as food, from the insipidity of its taste. 



SECTION R ARCACE^E. 

 1034. The Area is distinguished b'y its equivalve shell, and 



FIG. 693. ARCA BARBATA AND ARCA NO.B. 



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

 minute teeth. The valves, which are covered with a strong 

 epidermis, do not always meet in the middle ; but a space is often 

 left for the passage of a horny, apparently tendinous substance, 

 which takes the place of the byssus, arid by which the animals 

 are affixed to submarine bodies. They reside near the shore in 

 rocky places. Numerous species are distributed in all parts of 



