FAMILY 12. NAIADES. 121 



an extended series of specimens, the relative value of these distinctive 

 characters is much reduced ; we find, for instance, that in some of the 

 shells of Anodon there is a slight indication of teeth, whilst in others of 

 the genus Unio the hinge is nearly edentulate. 



We describe the shell of Anodon as being transverse, equivalve, in- 

 equilateral, generally thin, covered with a green epidermis, and some- 

 what pearly within. In the young state the shell is rather depressed, 

 but it increases in convexity as it increases in age. The hinge is linear, 

 edentulate or destitute of teeth, with a long external ligament, and the 

 impression of the posterior muscle is always compound. 



Example. 

 PI. XCI. Fig. 1 and 2. 



Anodon anatinus, Lamarck, Anim. sans vert., new edit., vol. vi. p. 565. 



Sowerby, Genera of Shells, No. 17. Pennant, Zool. Brit., pi. 68. 



f. 79. 

 Mytilus anatinus, Linnaeus. 

 Unio anatinus, Deshayes. 



IRIDINA, Lamarck. 



Testa transversa, subcylindrica, sequivalvis, insequilateralis, epidermide 

 olivacea induta, interne rubro-margaritacea, plus minusve irides- 

 cens. Cardo linearis, praelongus, versus medium subattenuatus ; 

 edentulus, sed tuberculis ineequalibus subcrenatis crebris per lon- 

 gitudinem instructus ; ligamento externo, marginali. Impressio 

 muscularis postica semper composita. 



The Iridinse are, of all the Naiades, most entitled to a generic distinction. 

 In the animal of the preceding genera the hinder part of each lobe of the 

 mantle is simple and free ; but in this genus it is united, and so pro- 

 longed as to form two tubes or siphons. 



vol. i. R 



