28 UNIONID.E. 



one, shall not consider unreasonable, although my pre- 

 sent impression is that they ai-e 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. teti'agonurn of Normand and the P. arccefonne 

 of Malm. 



As some test of specific distinction, I w^ould remark 

 that the following species of Pisidium are often found 

 li\dng together : viz., amnicum Sindfontinale (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 Sphosrium. 



Family 11. UNIONIDiE. 



13oDY oblong, compressed : mantle open on all sides except 

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

 w^liich correspond with the cylinder or tubes of the Sphceriidce. 

 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 cut! 

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

 broad, and tongue-shaped. 



Shell equivalve, oblong, inequilateral, compressed : epi- 

 dermis thick : heahs (which form the nucleus or yoimg shell) 

 plaited or wrinkled: ligament external, strong, and always 

 conspicuous : imide pearly : himje furnished with lateral teeth 



