The Mollusca of the estuarine region of the rivers Rhine, Meuse, and Scheldt in relation to the hydrography of the area. V. The Cardiidae

Based on literature data and personal investigations it is concluded that Cerastoderma edule (L.) and C. glaucum (Poiret) are closely related, but distinct species. The haploid chromosome number of both species is 19. The distribution and ecology of the two species in the estuarine area of the rivers Rhine, Meuse and Scheldt are described in relation to environmental factors. C. edule lives nearly exclusively in tidal environments, C. glaucum in areas with stagnant water. This is in accordance with the hypotheses that C. edule cannot live in stagnant water, owing to a detrimental substance liberated by the living animals themselves, and that C. glaucum does not tolerate wave-action or a long period of emersion. It is concluded that C. edule shows a correlation between mean rib number and mean salinity, whereas C. glaucum does not. Therefore Eisma’s (1965) method for determining the salinity of fossil seas is modified.