DREISSENID^S. 207 



between some species of Unio, and still more in the 

 case of Anodonta. 



A. cygnea has the body gray, with yellowish or 

 reddish tint, the mantle bordered with , brown, the 

 foot broad, dirty yellow lined with orange or red, the 

 gills gray with a gauzy texture. The shell is oblong, 

 yellowish-green, thin, moderately glossy, transversely 

 and irregularly grooved by the lines of growth, 

 wrinkled in the same direction on the posterior and 

 lower sides ; epidermis thin, beaks straight, ambonal 

 region compressed, strongly plaited ; ligament rather 

 long, strong, and partly concealed, the inside pearly- 

 white and iridescent. 



Of this species there are several varieties, all very 

 distinctly marked, and by older authors called species. 

 The length is about 275, the breadth about 5-35. It 

 is an Upper Tertiary species. The young have tri- 

 angular pearly shells, easily confused with Cypris or 

 smaller entomostraca. 



A. raticeta is 2*1 long, and 3*5 broad. The shell 

 is oval, compressed, but not so thin as the last-named 

 species; the colour is olive-green or brown. The 

 epidermis is thicker than in A. cygnea. 



The family of Dreissenida contains but one genus. 

 The shell is equivalve, oblong, triangular, inequilateral, 

 ventricose, and covered with a horny epidermis, 

 the beaks are at the anterior end, the ligament is 

 internal, and the inside porcelain-white; the hinge 

 has sometimes small cardinal teeth. Below the beak 

 in each valve is a hollow plate for the reception of the 

 anterior muscle. 



The genus Dreissena, which may be called the 



