Nayades.] 



MOLLUSCA. 



7H 



1. Anodon Cvgneus. 



Anodonta Cygnea, Lamarck, An. San. Vert., VI, p. 84; Dra- 

 pernaud, Hist. Nat. des Moll., p. 134, pi. 1 1, f. 6, and pi. 12, f. 

 1; Anodon Cygneus, Turton, Brit. Biv., p. 239; Mytilus Cyg- 

 neus, Linnaeus, Syst. Nat., p. 1 158; Gmelin, p. 3355; Montagu, 

 Test. Brit., p. 170. 



Shell thin, oblong-ovate, tumid; anterior side short, rounded; 

 posterior slope elongated, and somewhat pointed, more or less 

 angulated, and subcompressed ; umbones but slightly produced, 

 and placed nearest the anterior side. 



This species is subject to great variety of form. These 

 varieties have, by different authors, been considered distinct 

 species ; but we have not been able to detect any specific dif- 

 ference in the animal inhabitant. There are, however, several 

 well-marked and permanent varieties, the distinguishing charac- 

 teristics of which we shall endeavour to point out. Most of 

 the Anodons undergo considerable change, in their progress 

 from the young to the adult condition. 



Variety 1. Cygneus, pi. XXVIII, f. 1, 3, 4. 



Anodonta Cygnea, Pfeiffer, 1, p. ill, pi. 6, f. 4; Rossmassler, 

 I, p. 1 1 1, pi. 3, f. 67 ; lb., V, VI, pi. 25, f. 342; Mytilus Cyg- 

 neus, Pennant, Brit. Zool., IV, p. 113, pi. 67, f. 78; Donovan, 

 Brit. Sh., pi. 55 and 1 13; Maton and Rackett, Linn. Tr., VIII, 

 p. 109, pi- 3 a, f. 2 ; Sheppard, Linn. Trans., XIII, pi. 5, f. 3 ; 

 Montagu, Test. Brit., p. 170; Fleming, Edin. Ency., VII, p. 96, 

 pi. 205, f. 16; Thompson, Ann. and Mag. Nat. Hist., VI, p. 40; 

 Anodon Cygnea, Alder, Mag. Zool. and Bot., II, p. 117. 



Shell rather thin, oblong-ovate, inflated; umbo small, placed 

 near the anterior side, which is rounded and short ; posterior 

 side elongated, with the hinge line nearly parallel, or slightly 

 oblique, from whence it suddenly slopes into a subrostrated 

 form; lower margin rounded, gradually sloping upwards towards 

 the posterior side ; external surface concentrically wrinkled, 

 and covered with an olive-green, shining epidermis ; inside 

 pearlaceous. 



Figs. 3 and 4, young shells. 



Found in the river Ouse, at York; the Thames, near Wind- 

 sor; in the ponds at Elmingham Park, the seat of the Countess 

 of Dysart, Suffolk ; Loch Kettrine, Scotland ; in the Grand 

 Canal, and the Moyntaghs, County of Armagh, Ireland ; and 

 many other lakes and rivers in Britain and Ireland. 



Variety 2. Cellensis, pi. XXVIII,* f. 1, 2. 



Anodonta cellensis, Encyclopedia Methodique, pi. 202; Pfeif- 

 fer, I, p. 110, pi. 6, f. 1 ; Rossmassler, IV, p. 23, pi. 19, f- 280; 

 Anodon cellensis, Alder, Mag. Zool. and Bot., II, p. 118; Ano- 

 don Cygneus, Drapernaud, Moll., pi. 12, f . 1 ; Turton, Man., 

 pi. 1, f. 8; Anodon paludosus, Turton, Brit. Biv., p. 240, pi. 

 15, f. 6; Anodonta sulcata, Lamarck, An. San. Vert., V, p. 85; 

 Mytilus cellensis, Schroeter, Flussconch., pi. 2, f. 1 ; Mytilus 

 anatinus, Pennant, Brit. Zool., IV, p. 1 13, pi. 68, f. 79; Maton 

 and Rackett, Linn. Tr., VIII, p. 110, pi. 3 a, f. 1 ; Montagu, 

 Test. Brit., p. 171 ; Sheppard, Linn. Trans., XIII, pi. 5, f.°5 ; 

 Fleming, Edin. Ency., VII, p. 99, pi. 204, f. 14. 



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 bein» 

 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. XXIX, f. 1. 



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

 85 ; Pfeiffer, I, p. 1 12, 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. 1 18. 



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. 

 It is considerably more inflated than the ordinary form of 

 Anatinus, and is of a small size. Its length is one inch and 

 three-quarters ; its breadth three inches and a quarter ; and its 

 thickness one inch and one-eighth. 



Found at Southport, Lancashire; Loch Leven, Kinrossshire ; 

 Prestwick Carr, Northumberland, where we obtained the beau- 

 tiful and highly characteristic specimen from which our figure 

 was taken ; and also in the rivers Shannon and Bresna, and the 

 Grand Canal, Ireland. 



Variety 4. Stagnalis, pi. XXVIII, f. 2. 



Mytilus stagnalis, Sowerby, Brit. Miscell., pi. 16; Mytilus 

 Cygneus, Maton and Rackett, Linn. Tr., VIII, p. 109; Mon- 

 tagu, Test. Brit., p. 171. 



Shell thin, much inflated; umbones subcentral; anterior side 

 rounded, with a slight angle above; hinge line arcuated; poste- 

 rior side slightly hollowed, and terminating in a subacute beak ; 

 basal line nearly semicircular ; outer surface with an olivaceous, 

 shining epidermis ; very pearlaceous and iridescent internally. 



In the younger condition, the colour is of a bright yellowish- 

 green. 



Found in the pond of Kew Garden ; in the Canal Wharf at 

 Bolton Bridge, Lancashire ; and in the canal at Keighley. 



Variety 5. Intermedia, pi. XXX, f. 5, 6. 



Anodonta intermedia, Lamarck, Hist. An. San. Vert., VI, p. 

 86; Pfeiffer, I, p. 113, pi. 6, f. 3; Alder, Mag. Zool. and Bot., 

 II, p. 118. 



