British species of Cyclas and Pisidium. 293 



Testa corticata, subovalis, insequilateralis. Dentes cardinules minuti ; 

 in dextra valvula unicus plus minusve complex ; in sinistra plerumque 

 duo. Dentes laterales longitudinales, compressi, lamelliformes, in valvula 

 dextra duplicati. Ligamentum externum, posticum, lateri breviori insi- 

 tum. 



Although Pfeiffer has the merit of having first separated the 

 above genera, his characters are not clearly defined, from the 

 circumstance of his having confounded the anterior and posterior 

 parts of the shell. The extremity from which the siphonal tube 

 is protruded, and which strictly speaking is the posterior, he has 

 termed differently in his descriptions of the two genera. I have 

 adopted the views of Blainville with reference to this point, as 

 well as in the selection of terms employed in characterizing the 

 species, and accordingly it is to his "Manuel de Malacologie " 

 that I must refer for an explanation of all such as occur in the 

 following paper. The best characters for distinguishing the above 

 genera are drawn from the structure of the siphonal tube, and the 

 position of the hinge considered relatively to the two extremities 

 of the shell. In the genus Cyclas, the tube is capable of protru- 

 sion to a considerable length, and although single at the base, is 

 always divided at the apex, the upper portion, which is generally 

 the shorter of the two, acting as the anus, while the lower, which 

 is both longer and has a larger aperture, serves to conduct the 

 water to the branchiae. In Pisidium, this tube is single and un- 

 divided throughout its whole length, and although to a certain 

 degree admitting of extension and contraction, is never protruded 

 to the distance that it is in Cyclas. Indeed in both genera it 

 appears to be quite a matter of pleasure whether it be exserted 

 at all ; the animal being often seen, both at rest and in motion, 



