114 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



Family 12. NAIADES 



Testa imequilateralis, valvis superne aut connatis aut liberis ; epidermide 

 olivacea induta, umbonibus plus minusve erosa. Cardo irregularis, 

 dentibus plerumque crassissimis duobus aut pluribus ; interdum 

 edentulus, interdum tuberculis fere obsoletis solummodo instruc- 

 tus. Animal fluviatile. 



The family of the Naiades includes a numerous and characteristic 

 group of fluviatile Tropiopoda, chiefly inhabiting the great rivers of 

 America. They have been variously classified ; but almost every at- 

 tempt to effect a natural division of genera has failed to a certain extent 

 on account of the continual discovery of new varieties, showing the gra- 

 dual modification that extends from one genus to another. Thus we find 

 ourselves as much puzzled in determining the generic distribution of the 

 Naiades, as we were in deciding that of the marine Conchaeea. 



De Ferussac has devoted particular attention to the arrangement of 

 this extensive family ; but upon considering the labours of the American 

 conchologists, who, from the local advantages they possess of observing 

 the Naiades in their native condition, are peculiarly entitled to notice, 

 we cannot but appreciate the pains, and very careful exposition made of 

 them by Lea. After dividing the family into two genera, Margarita and 

 Platiris, according to the characters of the animal, he adopts as subge- 

 nera of the first division. Unio and Anodon of Bruguiere, Margaritana of 

 De Ferussac, Dipsas of Leach, and Pleiodon of Conrad ; and the second 

 division, which answers to Lamarck's genus Iridina, is divided into two 

 genera, Iridina and Spatha. He then separates the symphynote Naiades, 

 or those with connate shells, from the nonsymphynote, or those with free 

 shells, the former being distinguished as connate on account of the pro- 

 longed growth of each valve behind the umbones becoming connected, as 

 in the Unio alatus (Plate LXXXIX. Fig. 8.). This peculiarity of growth 



