AND DESCRIPTIONS OF NEW SPECIES. 67 



When I established the genus Symphynota*, I remarked on the 

 difficulties attending the present generic divisions of the family Naiades. 

 Since that period a closer attention to these divisions has convinced 

 me of the entire impossibility of defining limits to them. The 

 hinges in the species of the different genera glide or shade away so 

 completely into each other, that I have no hesitation in saying it is 

 entirely impossible for any naturalist to mark out a line of unvarying 

 character to most of them. It must therefore be conceded that other 

 characters are required for generic divisions. 



If w^e examine the 



Jlnoilonta cygnea (Lam.), we find the margin under the beak and 

 ligament to be an iminterrupted line. In the 



Iridina nilotica (Sowerby) this line is slightly interrupted under the 

 point of the beak. In the 



Anodon areolatus (Swainson) we have this interruption more 

 distinctly marked, the elevations being larger and more curved, evi- 

 dently forming an incipient tooth which approaches very closely to the 



Alasmodonta marginata (Say), and forms with it a natural link. 

 The next in the chain appears to be the 



.fllasmodonta rugosa (Barnes), which has an incipient lateral tooth ; 

 and that which follows very closely is the 



Unio calceolus (Nob.), which has the lateral tooth very slightly more 

 defined than the preceding. In the 



Symphynota compressa (Nob.), we have the tooth more perfect and 

 extended, forming a moderately well characterised lateral tooth of the 

 genus Unio. The well known 



Unio pictorum (Mya pictorum, Lin.) presents us with cardinal and 

 lateral teeth completely formed. In this genus, the Unio, we have 

 an infinite variety in the forms of teeth. In the 



Symphijnota alata (Nob.), the cardinal and lateral teeth are com- 

 pressed in most specimens ; and the next change we find, is in the 



Hyria avimlarisf (Lam.), in which the cardinal tooth is somewhat 



* See Vol. III. p. 442. 



t When the animal of tiiis genus shall be e.xamincd, it will be found, I have no doubt, to 

 differ from the Unio, Anodonta, &lc. For notwithstanding Lamarck's description, " elles ont 

 interieurement les impressions musculaires laterales des Nayades," 1 have discovered that the 



