68 OBSERVATIONS ON NAIADES, 



lamellar and forms nearly a line with the lateral tooth. The next 

 " nuance" is in the 



Si/mphynotal3evissima(Noh.), which possesses lamelliform cardinal 

 and lateral teeth forming nearly a complete arc. Then follows the 



Symphynota bialata (Nob.), the uninterrupted curved tooth of which 

 is little more than an elevated line under the ligament and beaks. As 

 far as one may be able to judge from a bad description and very bad 

 drawing, the 



Dipsas plimtus (Leach) may be with propriety placed at the end of 

 this suite. 



In the U. oriens described in this paper, we have a peculiarity in 

 the formation of the termination of the lateral tooth, which is en- 

 larged. 



Under the impression, therefore, that the teeth in the Family iVam(/e« 

 do not form a sufficient distinctive character to compose genera, I 

 propose to make a "division" of the family, the distinctive character 

 of which will be valves free and valves connate ; the genus Unio to 

 include the first, the genus Symphytiota to include the last. If subse- 

 quent groups be necessary, these may be composed of subgenera. 



In my catalogue* of species, which I presumed should be considered 

 as established, I gave the umlulatus of Barnes as a synonym, consider- 

 ing it as a variety of the plicutus. Conversing with that naturalist 

 over his cabinet some time before his death, he expressed himself as 

 being very much of " that opinion. At that period neither of us had 

 seen a young specimen of this species ; very recently I have been for- 

 tunate enough to obtain several, and the examination of the beaks of 

 these, which are nearly perfect, convinces me, that although the umlu- 

 latus resembles the plicatiis in its general characters, yet, that the 

 beaks are sufficiently dissimilar to make them specifically different. 



extensor muscle of the foot is attached to the internal base of the cardinal tooth and there 

 forms a remarkable cicatrix, which of course is over the large anterior (posterior of Lam.) 

 cicatrix, while in all the numerous species of Naiades which I have examined, the cicatrix of 

 the extensor muscle has been situated below the large anterior cicatrix. In anatomical struc- 

 ture they must tiierefore differ. 

 * Transactions, Vol. III. 



