16 SPH^RIID^. 



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 concluded by saying that he found the number 

 of filaments to vary from one to at least four, which in 

 one instance were far apart, the siphons or tubes and 

 foot being at the same time exposed ; and that the 

 animal had the power of raising itself by means of this 

 byssus again to the surface, after having been suspended 

 for some time below it. The filaments appeared not to 

 exceed half an inch in length ; and rarely could more 

 than a single thread be seen. M. Bouchard-Chante- 

 reaux has likewise, in his extremely interesting Cata- 

 logue of the Land and Freshwater Mollusks which in- 

 habit the Department of the Pas-de- Calais, noticed that 

 the young of S. corneum possess the same faculty of 

 spinning a transparent thread and attaching themselves 

 by means of it to water-plants. 



Genus II. PISI'DIUM ^ C. Pfeifi"er. PL I. f. 3, 4. 



Body inequilateral : mantle having only a single tube. 



Shell inequilateral : beaks placed near the shorter or an- 

 terior end. 



This genus was established by Carl Pfeiflfer in 1821, to 

 separate from Sphcerium the smaller species which have 

 only one tube or siphon, and whose shells are not so 

 equilateral. This generic distinction seems to be well 

 founded, in respect both of the soft parts of these mol- 



* Pea -shaped. 



