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OVAL. 



*cygnea. x Drap. Lam. Mill. Crouch. 



Blain. Pfeif. Turt. Des Moul. 



Flem. Grat. Bouil. Brard. Bose. 



Children. Puton. Potier. Gassies. 



Por. Klees. Morelet. Merm. Fori). 



and Hani. Villa. Guerin. Graells. 



Thompson. Cuv. Gray. Kiist. 



Brown. Bouch. Rossm. Desh. 



Studer. Menke. Gras. Goupil. 



Dup. Fitz. Swain. Schrenck. 



Charp. Waard. Kickx. Midd? 



Gras. Stein. Chenu. Brot. Bourg. 



Han. Moq. Caill. 

 Myt. cygneus. Lin. Chem. Schro. 



Dill. M'uhl. Monta. Mat? Shep. 



Tur. Mull. Sehreib. Pennant. 



Don. Da Costa. 

 Myt. anatinus. Lin. Chem. Schro. 



Schum. Wood. Monta. Don. Da 

 Shep. Tur. Dill. 

 Grat. 

 Gmel. 



Costa. Penn. 



Mat. Sehreib. 

 Myt. stagnalis. 



Sehreib. 

 Myt. fluviatilis. 4 



Dill. Sow. 



Gmel. Sehreib. 



f OVAL. 



Myt.fucatus. Dill. 



Myt. Zellensis. Gmel. Schro. Scltreib. 



Myt. Avonensis. Monta. Wood. Ed. 



Encyclopaedia. Moq. 

 Myt. radiatus? Mid. Schro. Sehreib. 

 Myt. incrassatus. Shep>. 

 Myt. maculata. Shep. 



fragilis. Klein. 



Musculus fluviatilis. Klein. 

 Musculus latus. List. Klein. 

 Musculus angustior. Klein. 

 Grand Moule des etangs. Geoff. 

 An. anatina. Lam. Cuv. Mill. Dill. 



Bosc. Drap. Soiv. Pfeif. Flem. 



Grat. Des Moid. Stud. Bouil. 



Macgil. Phili. Menke. Desh. 



More. Goup. Nils. Put. Kiist. 



Dup. Aid. Graells. Villa. It lees. 



Por. Gass. Bouch. Pot. Charp. 



Drouet. Waard. Fitz. Arada. 



Midden. 6 Gerst. Brot. Bourg. 



Han. Moq. Caill. 

 An. sulcata. Lam. Nils. Vill. 

 An. dentiens. Menke. 



1 I have, after a good deal of consideration and examination of my specimens, and the figures in 

 the numerous works describing the Unionidse, satisfied mj'self that An. cygnea and An. anatina are not 

 specifically distinct. If the observation of M. Poiret, that the first is viviparous and the last oviparous, 

 be correct, then they should be certainly separated. I feel perfectly persuaded, however, that he must 

 be in error. Turton, in his recent work on the Land and Fresh Water Shells of Great Britain, says he 

 is "inclined to think that all our supposed species of this genus may be justly resolved into one." M. 

 Gras (Description des Moll. Fluv. et Ter. de la France) sa3 r s that anatinus is only the young of cygneus ; 

 and Dr. Gray says, in his Manual of Fresh Water and Land Shells, that it is a most variable species, and 

 that "we must not only dissent to the division of this polymorphous bivalve into these numerous species, 

 into which it has been separated by the continental writers, but even demur to the possibility of arrang- 

 ing the diversities of shape and coloring into strictly different varieties." "Like Unio, this genus is 

 chiefly American, only one distinctly-marked species inhabiting Europe." Such has been my opinion for 

 more than twenty years. (See note on Unio pictorum.) 



2 Middendorff (Sibi. Reise) refers to his PI. 21, Figs. 4 and 5; PI. 22, Figs. 1 and 2; and PI. 26, 

 Figs. 1 and 2, for An. cygnea, but Figs. 1 and 2 on PI. 22 are undoubtedly Dipsas plicatus, Leach. 



3 £ of Maton and Racket (Lin. Soc. Trans., vol. iv.) is evidently, judging from the figure, Unio 

 litoralis. 



4 Gmelin states this shell to be from the fresh waters of Europe, and allied to Anatina. If this be 

 true, there cannot be a doubt of its being the same with cygnea. The fluviatilis of Solander and Dillwyn 

 is said to be from North America, and Say's cataracta is the same, no doubt. 



5 Fide Dillwyn. 



6 Tunguska River, Siberia. 



