|External view of the left valve of Pisidium pseudosphaerium|
C. Pfeiffer, 1821
Tetragonocyclas Pirogov & Starobogatov, 1974
Pisidium is a genus of very small or minute freshwater clams known as pill clams or pea clams, aquatic bivalve molluscs in the family Sphaeriidae, the pea clams and fingernail clams.
In some bivalve classification systems, the family Sphaeriidae is referred to as Pisidiidae, and occasionally Pisidium species are grouped in a subfamily known as Pisidiinae.
In large enough quantities, the minute shells of these bivalves can affect environmental conditions, and this change in conditions can positively affect the ability of organic remains in the immediate environment to fossilize (one aspect of taphonomy). For example, in the Dinosaur Park Formation, the fossil remains of hadrosaur eggshells are rare. This is because the breakdown of tannins from the local coniferous vegetation caused the ancient waters to be acidic, and therefore usually eggshell fragments dissolved in the water before they had a chance to be fossilized.
Hadrosaur eggshell fragments are however present in two microfossil sites in the area. Both of these sites are dominated by preserved shells of invertebrate life, primarily shells of pisidiids. The slow dissolution of these minute bivalve shells released calcium carbonate into the water, raising the water's pH high enough that it prevented the hadrosaur eggshell fragments from dissolving before they could be fossilized.
Drawing of the right valve external view of Pisidium moitessierianum
Drawing of the right valve internal view of Pisidium moitessierianum
Drawing of the right valve lateral view of Pisidium moitessierianum
SEM image of Pisidium sp.
Extant subgenera and species within the genus Pisidium include:
Subgenus Euglesa Jenyns, 1832
Subgenus Pisidium Pfeiffer, 1821
Subgenus Cyclocalyx Dall, 1903
Subgenus Tropidocyclas Dall, 1903
Subgenus Hiberneuglesa Starobogatov, 1983
Subgenus Cingulipisidium Pirogov & Starobogatov, 1974
Subgenus Pseudeupera Germain, 1909
Subgenus Neopisidium Odhner, 1921
Subgenus Odhneripisidium Kuiper, 1962
Subgenus Afropisidium Kuiper, 1962
Subgenus incertae sedis