LAMELLIBRANCHIATA : UNIONIDyE. 49! 



about twelve fathoms of water. There are about one 

 hundred living and six hundred fossil species. Fig. 453. 

 MYTILID.E, OR SEA-MUSSEL FAMILY. - -This Family 

 comprises acephala which have the shell equivalve, oval, 

 or elongated, and the epidermis thick and dark. They 

 seek concealment, and spin a nest of sand, or burrow in 

 mud-banks. There are more than one hundred living, 

 and two hundred and fifty fossil species. Fig. 452. 



Fig. 453- F 'S- 454- 



Avicnla hi r undo, L. One half. 



Mediterranean. Leda limatula. New England. 



ARCAD.E. - - This Family embraces acephala which 

 have the shell regular, with a strong epidermis, and the 

 hinge with a row of comb-like teeth. There are three 

 hundred living, and six hundred fossil species. Fig. 454. 



TRIGONIAD^E, OR TRIGONIA FAMILY. - -This Family 

 contains acephala which have the shell trigonal, and the 

 hinge teeth few and diverging. There are three living 

 species, and one hundred and thirty or more fossil. 



UNIONID/E, OR POND AND RIVER MUSSEL FAMILY. 

 This Family embraces a large number of bivalves which 

 are found in ponds, brooks, rivers, and fresh-water lakes. 

 They are the N aides of authors. Figs. 456-460. 



CHAMHXE. -This Family comprises acephala which 

 have the valves unequal, thick, two hinge teeth in one 

 valve, and one in the other. Fifty living species. 



TRIDACNID.E. This Family comprises very large bi- 

 valves of the Indian and Pacific Oceans. The shell some- 

 times weighs five hundred pounds, the animal twenty. 



CARDIAD^E, OR COCKLE FAMILY. This Family em- 

 braces acephala which have the shell cordate and orna- 

 mented, with radiating ribs. There are two hundred liv- 

 ing, and three hundred fossil species. Marine. Fig. 455. 



