28 UNIONISE. 



one, shall not consider unreasonable, although my pre- 

 sent impression is that they are distinct species. The 

 variation of form and sculpture is undeniably very great 

 in all freshwater shells ; and this is probably caused not 

 only by the greater or less supply of food procurable 

 by these mollusks, but also by the chemical ingredients 

 of the water from which their materials are secreted or 

 extracted. Development of size, and of particular por- 

 tions of the shell (by which its shape is determined), 

 seems to depend on the former condition, while its 

 solidity and sculpture are affected by the nature of the 

 fluid which these mollusks inhabit. The present species 

 is the P. ietragonum of Normand and the P. arcatforme 

 of Malm. 



As some test of specific distinction, I would remark 

 that the following species of Pisidium are often found 

 living together : viz., amnicum &nd fontinale (var. Hens- 

 lowana) ; fontinale and pusillum ; and fontinale (var. 

 pallida), nitidum, and roseum. Each of the above is also 

 sometimes found solitary, or in company with various 

 species of Sphcerium. 



Family II. UNIONISE. 



BODY oblong, compressed : mantle open on all sides except 

 at the back, but forming at the posterior side two orifices, 

 which correspond with the cylinder or tubes of theSphceriidce. 

 The smaller and upper, or excretal, orifice is separated from 

 the larger and lower, or branchial, orifice by an intermediate 

 fold of the mantle. The margin of the first-mentioned orifice 

 is plain; but the other is fringed with several rows of cirri 

 or tentacles. Mouth placed as in the last family. Foot large, 

 broad, and tongue-shaped. 



SHELL equivalve, oblong, inequilateral, compressed : epi- 

 dermis thick : beaks (which form the nucleus or young shell) 

 plaited or wrinkled : ligament external, strong, and always 

 conspicuous: inside pearly: hinge furnished with lateral teeth 



