54 Sowerby on the Lamarckian Naiades. 



between these genera, and that the transition from one to another 

 is so gradual in some instances, and so strongly marked in others, 

 that it is not surprising that authors who having only met with cer- 

 tain species, and not being aware of such intermediate links, 

 should have considered them as the types of new genera. That 

 this has been the case with Lamarck's Iridina, is abundantly 

 proved, as we think, by the species which we have published in 

 the present number, which being destitute of the remarkable cre- 

 nulations that according to Lamarck form the principal character of 

 that genus, should have been considered as an Anodon, did it not 

 possess all the other peculiarities of Lamarck's Iridina exotica, and 

 of our I. elongata. Such characters should not therefore be ele- 

 vated to the rank of generic distinctions. Alasmodontct* of Say, 

 Dipsas of Leach, Hyria and Castalia of Lamarck, are all sepa- 

 rated from Unio, as we are about to demonstrate, upon characters 

 of no more importance, because upon such characters we should 

 be warranted in raising almost every strongly marked species into 

 a genus. 



It is remarkable that all the authors we have consulted have ob- 

 served how nearly related all the genera that have hitherto been 

 formed of the Naiades are, and also the similarity of their ani- 

 mals ; Lamarck particularly observes that the Anodontes are so 

 nearly related to the Uniones that they could not be distinguished 

 except by the characters of the hinge ; but we shall see that there 

 are Naiades not only without teeth, as Anodon ; with a crenulated 

 hinge line, as Iridina ; with wings and elongated hinge teeth and 

 lateral teeth, as Hyria ; and with short hinge teeth and lengthened 

 lateral teeth, as Unio : but also that there are others, with small 

 undulations at the hinge, (Unio Anodontina Lam. Anodon rugo- 

 sus Swains.) intermediate between Anodon and Unio: with scarcely 

 any appearance of crenulations along the hinge Hue, (Iridina Ni- 

 lotica) intermediate between Anodon and Iridina : with hinge 

 teeth only, ( Alasmodonta of Say and Unio glabratus ? Sow.) inter- 

 mediate between Unio and Anodon : with a large wing and no 

 teeth, (Anodon alatus. Sow.) between Hyria and Anodon : with 

 a wing and short hinge teeth, (Unio alatus auct.) between Unio 

 * More correctly Alasmodon. 



