Mr. Jeffreys on British MoUusca. 37 



fibres — even a single stem is quite sufficient ; and if tlie shell is free 

 from any other contact, it immediately begins its rotatory movement. 

 A single shell, thus suspended, revolves upon its axis in a direction 

 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. 

 C. cornea is much less active or inclined to ascend the glass ; in fact, 

 I have not yet seen it accomplish the feat of its congener. Several 

 of the C. calyculata will remain among the stems of the duckweed 

 for hours perfectly inactive, with closed valves, as if sleeping or rest- 

 ing 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 adliesion to the glass." Dr. 

 Lukis now informs me that he has detected the byssal filament in 

 Sph. calyculatum. He says, "I have this morning (16th Dec.) 

 watched one, which had reached the surface, spin its filament, and 

 descend to half an inch below the surface, where it remained sus- 

 pended for some time. It occupied three hours in spinning this 

 short thread. I think it consists of more than a single filament, for 

 some minute particles, which were floating in the water, became 

 entangled in it. The surface of the water was, again, depressed or 

 cupped.'* And he concludes by saying that he found the number 

 of the fibres varies from one to at least four, which in one instance 

 were far apart, the siphons and foot being at the same time exposed ; 

 and he adds that the animal has the power of raising itself by means 

 of this byssus again to the surface after having been suspended below 

 it for some time. The filaments appeared not to exceed half an inch 

 in length, and rarely more than one thread was distinctly visible. 



Pisidium. Having examined the original and typical specimens 

 of Mr. Jenyns, which he most obligingly sent me for that purpose, 

 and being aided by a critical investigation of the species which I 

 have carried on for very many years in this country, as well as in 

 France, Germany, and Italy, I am inclined to reduce all the British 

 species to the following: viz. 1. pusillum (including ohtusale) \ 

 2. nitidum ; 3. roseum ; 4. Henslowianum (^nclxxdim^ pulchellum and 

 cinereum, the types of which last I have also examined, through the 

 kindness of Mr. Alder) ; and 5. amnicum. The variation of form, 

 striation, and comparative solidity is imquestionably greater among 

 freshwater than marine shells ; and it is probably owing to the dif- 

 ferent nature and qualities of the fluid which they inhabit and from 

 which their materials are secreted. 



P. pusillum, var. P. ohtusale (teste Jenyns), ii. 1 20. Clevedon 

 {Uev. Mr. Norman) ; Serk (/. G. J.). I have never met with this 

 variety in company with the typical form. Its habits appear to be 

 more active, owing perhaps to the difl*erence of the water and locali- 

 ties in which it is found. 



Ditto (typical form), ii. 123. A very fine variety, of a paler colour, 

 occurred to me some time ago in the neighbourhood of Llanelly, 

 Carmarthenshire ; and Mr. Bean has lately sent me the same variety 



