20 SPHiERIIDJJ. 



A. Triangular. 



1. PisiDiuM am'ntcum*, Miiller. 



Tellina amnica, Midi. Verm. Hist. p. 205. P. amnicum, F. & H. ii. p. 133, 

 pi. xxxvii. f. 8, 9, and (nnimal) pi. O. f. 8. 



Body greyish-white, rather transparent : tube short, sub- 

 conical, obliquely truncate at its orifice : foot broad at its 

 base, abruptly pointed, and very extensible : mantle bordered 

 with grey. 



Shell subtriangular, rather ventricose and soUd, glossy, 

 strongly grooved concentrically, horn-colour or yeUowish- 

 grey: epidermis rather thick : anterior side abruptly truncate : 

 ■posterior side much produced, and sloping towards the lower 

 margin, which is obliquely curved : heahs rather prominent, 

 but obtuse : ligament short, conspicuous : inside bluish-white 

 and nacreous : hinge strong and curved ; teeth arranged as in 

 Sphcerium, but the lateral teeth in this and other species of 

 Pisidium are exceedingly strong and developed in proportion 

 to the size of their shells : muscular and pallial scars well 

 marked. L. 0-3. B. 0-375. 



Varieties occur in which the stride are more numerous, 

 fewer, stronger or fainter than usual. 



Haeitat : Slow rivers, lakes, canals, and streams in 

 aU parts of the kingdom. It is also one of our com- 

 monest upper tertiary fossils. Its continental range ex- 

 tends from Siberia to Naples, and it is also found in 

 Algeria. This is the largest kind of Pisidium. 



2. P. fontina'le t, Draparnaud. 



Cyclas fo7itinalis, Drap. Hist. Moll. p. 130, pi. x. f. 8-12. P. Henslowi- 

 anum (var. without appendages), Jenyns in Ann. Nat. Hist. Aug. 1858, 



Body whitish or grey, rather transparent : tube generally 

 short, but capable of considerable extension and dilatation, 

 conical, obliquely truncate at its orifice, where it is so flexible 

 that the edges are sometimes entire and at other times jagged : 

 foot long and curved : mantle bordered with grey. 



* Inhabiting rivers. f Inhabiting fountains. 



