268 Leds Si/?iopsis of the Naiades. 



of Naiades ; and, for some years past, the attention of American 

 naturalists has been particularly directed to the study of this 

 curious and interesting group of molluscs. The memoirs of 

 Mr. Lea upon the Uniones, and other fresh-water genera, con- 

 stitute a leading feature in the pages of the American Philo- 

 sophical Transactions; and, besides the distinguishing characters 

 of numerous species, we are indebted to the researches of this 

 conchologist for much valuable information upon the subject 

 of their animal economy. 



The difficulty attending the discrimination of the species of 

 Naiades, and the unsatisfactory nature of the attempts made to 

 point out decisive characters for the subdivision of this family, 

 have been acknowledged by conchologists when the individuals 

 composing it were thought to be considerably less numerous 

 than the number with which we are now acquainted, and to 

 which new forms are being constantly added. In those in- 

 stances where, as in the case before us, there occurs a large 

 number of very closely allied species, their separation into sub- 

 divisions, either by natural or artificial characters, becomes 

 almost indispensable, in order to facilitate the labours of both 

 the student and the experienced conchologist. To effect a 

 systematic and natural arrangement of the Naiades has been 

 an object of primary importance with Mr. Lea; and the 

 opportunities which he has had for pursuing this enquiry 

 must necessarily have been of a most extensive nature. 



The genus termed Symphynota, formerly established by 

 this author for the reception of those species in which the 

 valves were united by a testaceous connexion above the hinge, 

 is, in his present classification, regarded as a subordinate cha- 

 racter; and it is proposed to divide the Naiades into two 

 genera, Margarita and Iridina; the former of which compre- 

 hends five subgenera ; namely : TXnio, having a cardinal and 

 lateral tooth ; Margaritana, having one tooth (cardinal); Dip- 

 sas, having a linear tooth under the dorsal margin ; Anodonta, 

 having no teeth ; and Pleidon, having the dorsal line granulate. 

 Each of the first four of these is divided into symphynote 

 and non-symphynote shells ; and these, again, into plicate, 

 nodulous, spinous, and smooth shells; which subdivisions are 

 again separated, according to the form of their outline ; as 

 quadrate, triangular, oblique, &c. The species included in 

 the genus Iridina are comparatively few ; and the generic 

 character is founded upon the animal being furnished with 

 tubes. 



The whole number of species included in the Synopsis is 

 344; and of these about 250 belong to the subgenus LTnio. 

 Mr. Lea states that he has endeavoured to follow out Macleay's 



