FAMILY NAIADES. 



219 



The genus Unio — which name, it should be mentioned, is not the 

 feminine noun signifying union, but the masculine unio, a pearl, in 

 the sense used by Pliny, — has been always attributed to Eetzius. 

 Mr. Lea, in the last edition of his ' Synopsis' (p. xiii., note), states, 

 that on a careful examination of the scarce work ' Nova Testaceorum 

 Genera,' published in 1788, it seems to him that the genus belongs 

 to L. M. Philippson, the author of the 'Dissertation,' Eetzius being 

 the presiding officer of the institution where this thesis was presented. 



The British species of Unio are : — 



1. tumidus. Shell ovately wedge-shaped, glossy verdigris green, 



rayed on the posterior side. 



2. pictorum. Shell elongately oblong, fidvous olive, marked with 



concentric epidermic ridges. * 



3. margaritifer. Shell elongately kidney-shaped, often coni- 



pressly pinched about the middle, black, posterior elongated 

 tooth obsolete. 



1. Unio tumidus. Sioollen Unio. 



Shell ; ovately wedge-shaped, gibbously swollen, very inequilateral, 

 anterior side short, rounded, a little gaping in the region of 

 the foot, posterior side slopingly acuminated, valves concen- 

 trically fibrously ridged at intervals, olive green, glossy, tinged, 

 and rayed on the posterior side with verdigris, umboes con- 

 fluently xmdulately wrinkled, hinge composed of an erect 



