Sowerby on the Lamarckian Naiades. 53 



A ht. V. Description of a new species of Iridina. 



Ikidina Nilotica. D' Audebard. 



I. testa crassiuscula, transversim elongata, antice attenuate ; 

 postice producta, latiore ; margine cardinali subarcuato edentulo, 

 sub umbonibus irregulariter et obsolete subundulato : margine ba- 

 sali sub-emarginato. Tab. 2. 



Diam. 1-6. Leugth2-J. Breadth 5-6. 

 From Senaar. Mr. Cailliaad.* 



In general form the present shell very closely resembles I. elon- 

 gata, figured in Sowerby 's Genera of Shells, No. 7, but its hinge 

 margin is not crenulated or dentated. Our specimens of this valu- 

 able shell were sent to England by the learned author of the Mol- 

 lusques terrestres et fluviatiles. 



Shell rather thick, transversely elongated, anterior side atte- 

 nuated, posterior produced, broader ; hinge line rather arched, 

 without teeth, obsoletely and irregularly undulated beneath the 

 umbones : base slightly emarginate. 



* See Bulletin general des annonces et des nouvelles seientitiques, 10th 

 livr. p. 45. 



Art. VI. Some observations on the Lamarckian Naiades,* 

 and the propriety of uniting them all under one generic 

 name. By. G. B. Sowerby, F. L. S. 



The difficulty of ascertaining to which genus of Lamarckian 

 Naiades certain species belong, arises from the very general si- 

 milarity in form, colours, eroded umbones, compound muscular 

 impressions, and also in the structure and habits of the animals 

 which inhabit them : in fact, an examination of a sufficient num- 

 ber of species will prove that no dependance can be placed npon 

 the characters by which authors usually attempt to discriminate 



* Naiades, a term used in Botany, and therefore, perhaps, not convenient in 

 Zoology. 



