NAYADES. 101 



Shell considerably elongated, with numerous, concentric, and 

 rather deep wrinkles ; hinge line and base nearly parallel to 

 each other ; anterior side short ; posterior side very long ; the 

 upper and under sides both suddenly contracting into a sub- 

 rostrated form ; covered with a greenish, shining epidermis. 



This variety differs from the Cygneus, in its form being 

 much more lengthened, and in the hinge and basal lines being 

 both nearly straight, and almost parallel to each other. 

 Fig. 2 represents the young shell. 



This shell varies considerably in size in different localities. 

 It has been found in the pond of my friend Thomas Glover, 

 Esq., at Smedley Hill, near Manchester, measuring three inches 

 and a quarter in length, and six and a half in breadth. 



Found in the pond of the Infirmary, Manchester, and in 

 many other ponds in the neighbourhood ; Borrodale, West- 

 moreland; Loch Vennachar, Perthshire; the loch and ponds at 

 Duddingston, near Edinburgh ; and in the Grand Canal, near 

 Shannon Harbour, King's County, Ireland. 



This variety is more commonly diffused throughout Britain 

 than any of the others. 



Variety 3. Anatina, pi. XIII. 



Anodonta Anatina, Lamarck, An. San. Vert., VI, pt. 1st, p. 

 85 ; Pfeiffer, I, p. 112, pi. 6, f. 2 ; Rossmassler, V, VI, p. 57, 

 pi. 30, f. 417 to 420; Thompson, Ann. and Mag. Nat. Hist., 

 VI, p. 55; Alder, Mag. ZooL and Bot., II, p. 118; Brown, 

 Illust. Conch., p. 79, pi. 29, f. 1. 



Shell thin, subcompressed ; anterior side very short, and 

 rounded ; hinge line abruptly ascending into an acute angle, 

 and suddenly descends in a hollowed line to a somewhat trun- 

 cated beak, which is a little produced at its lower angle ; from 

 the anterior side the basal line suddenly descends in a rounded 

 form, and again makes an acute upwards angle, to meet the 

 posterior line; outer surface covered with a shining, fresh-green 

 epidermis. 



The posterior side of the shell is very long, and much 

 broader than in any of the former varieties. 



This variety is easily distinguished from its congeners, by the 

 posterior side being much broader than any of them, and from 

 the very abrupt upwards angle of the hinge line. A modifica- 

 tion of this variety occurs in the river Cam, near Cambridge. 



