Sphaerium cf. S. corneum (Linnaeus, 1758)
Known distribution: the whole Palearctic.
Suspension and deposit feeder.
Like S. nucleus and S. ovale, Sphaerium corneum bears numerous visibles pores. Because nucleus is not yet present in France, we have to decide between corneum and ovale for the present shells. As the density of pores is considered to be lower in corneum (about 80 pores per sq.mm) than in ovale (180-300 pores per sq.mm), one can imagine that the specimens pictured here belong to ovale (the density of pores on the images is much higher than 80 per sq.mm). But this density seems to be, in fact, rather variable, which makes this feature relatively useless for any identification based on the morphology only. So, as most of the other characteristics overlap from one species to the other, only one remains that could be of some use: the convexity of the valves. S. ovale is said to have moderately convex valves while corneum would be more globose. This is the reason why I propose to choose corneum as species name for these shells.
Above: a young shell collected in sediments at Font Blouze, Miers, Causse de Gramat, Lot, S. France. Altitude ≈ 315m. Size: 2mm. Details: 1mm wide images of 1) this specimen and 2) the one below.
