294 Mr Jenyns's Monograph on the 



with the tube either entirely concealed within the shell, or ex- 

 tended to its utmost limit. Another obvious distinction between 

 these two genera, is afforded by the position of the hinge and 

 cardinal teeth, with respect to the extremities of the shell. The 

 teeth themselves are similar in the two instances, at least in 

 general character; although subject to minute vaiiation of form, 

 more especially the cardinal tooth in the right valve, even 

 amongst individuals of the same species; but the situation of 

 the hinge is essentially different in Cyclas and Pisidium. In the 

 former genus it is nearly central, the posterior portion of the 

 shell being only to a slight degree longer than the anterior: in 

 the latter, it is placed towards one end, and the anterior portion 

 is obviously the longest; and although the excess of length in 

 the first section of this genus is not very considerable, yet it will 

 always be found in front of the hinge and not behind it. This 

 will be made to appear more clearly by inspecting Plate xix. in 

 which Fig. 3. represents the hinge of Cyclas calyculata, and 

 Fig. 4. that of Pisidium amnicum. In each case the right and 

 left valves are distinguished by the letters A and B respectively, 

 whilst a points out the relative position of the cardinal teeth. 



Having made the above observations upon the generic 

 distinctions afforded by these shells, I shall now proceed to 

 characterize the species themselves in the order of their respective 

 affinities. 



Gen. I. CYCLAS. 



Sp. 1. C. rivicola, Leach. 



C. testa globoso-ovali, ventricosa, striata ; umbonibus obtusis ; antice 

 lunula flava impressa ; ligamento cardinali conpicuo. 

 Long. 10^ lin. Alt. 8^ lin. Crass. 6^ lin. 



