CONCHIFERA OR BIVALVES. 101 



very large, thick, and of a square form. — 

 83 species recent, and two fossil. 



The Unto is found in the rivers of Europe, 

 both the Indies, and elsewhere ; it is called the 

 fresh-water mussel. It buries itself in the sand, 

 with the bosses undermost, and sometimes 

 produces tolerably large pearls: the valves in 

 some species are not quite closed, and they are 

 often highly iridescent within, sometimes white, 

 sometimes pink ; the colour, however, though 

 constant in some species, frequently varies in 

 different individuals of the same. 



60. Hyria. — Shell equivalve, obliquely tri- 

 angular; hinge mar- 

 gin straight, both 

 extremities elevated 

 and winged ; cardinal 

 tooth long, and di- 

 vided into many 

 parts, lateral tooth 

 long; ligament external. Animal resembling 

 that of the Unio. — 2 species recent. 



Found in exotic lakes, covered with an olive- 

 coloured epidermis, and pearly within. 



61. Anodonta. — Shell generally thin, equi- 



H 3 



I 



