SPH^RIUM. 15 



horizontally along the surface of the water, which ap- 

 pears to recede partially from it. On examining it with 

 a lens, the foot is distinctly seen to have an undulating 

 action on the surface, as well as an irregular and im- 

 perfect contraction and elongation along its whole ex- 

 tent ; but it is never quite retracted, excepting when its 

 base and the front margin of the valves are in contact 

 with some floating weed which is capable of supporting 

 the whole. Thus this elegant shell traverses the still 

 surface. But it is most curious to see it descending the 

 thread-like stems of the Lemna, or some assemblage of 

 these delicate fibres : even a single stem is quite suffi- 

 cient ; and if the shell is free from any other contact, it 

 immediately begins its rotatory movement. A single 

 shell, thus suspended, revolves upon its axis in a direc- 

 tion which is most frequently from right to left of the 

 observer, or in the opposite direction from that of a 

 teetotum when made to spin by the fingers of the right 

 hand. I have suspended single threads to circular pieces 

 of cork in the water ; but the stems of the Lemna are 

 preferred. Cyclas cornea is much less active or inclined 

 to ascend the glass ; in fact, I have not yet seen it ac- 

 complish the feat of its congener. Several of the C caly- 

 culata (Sphcerium lacustre) will remain among the stems 

 of the duckweed for hours perfectly inactive, with closed 

 valves, as if sleeping or resting after their previous 

 fatigue. When the valves are pressed against the glass 

 while ascending, there seems to be a fulness about the 

 base of the foot, as if the mantle served for adhesion to 

 the glass.^^ Dr. Lukis afterwards informed me that he 

 had detected the byssal filament in S. lacustre. He 

 says, '^ I have this morning watched one, which had 

 reached the surface, spm its filament j and descend to 



