Description of some species and subspecies of the genus Astarte from the Neogene of the Netherlands

Astarte pygmaea Heering, 1950 (non Von Münster, 1837), p. 75, pl. 1 figs. 7, 8. Shell small, softly shining, rather thick, rather convex, a little more broad than high. Ventral margin strongly and regularly curved. Dorsal margins nearly stretched, anterior side a little concave, posterior side a little convex. The anterior margin merges with a regular curve into the anterodorsal margin, the posterior side of the shell is weakly angular. Beaks prosogyrate, small, acute, not very convex. In well preserved shells a smooth, obliquely lanceolate prodissoconch is visible, its top lying at 2/3 of its length, length 0.6 mm, height 0.5 mm. Shell surface of adult shells covered by ca. 35 rather strong ribs, which are separated by narrow, but deep furrows. On the uppermost part of the beak the ribs are fine, not flat and closely set. On the middle part of the shell surface, however, the ribs are broad and flat, but they become quickly narrower in both directions towards the dorsal margins. Where the ribs are broad and flat, they are not parallel to the ventral margin, but are more strongly curved and sagging. In adult shells, however, the ribs become narrower, more regular and parallel to the ventral margin. Thus, only in juvenile valves the ribs run out from both sides of the ventral margin. From the top to the posteroventral margin a very weak depression in the shell surface is present, only visible under incident light. In this depression the ribs alternate, often very regularly, and new ribs start just between two ribs coming from the centre of the shell, but the ribs may also alternate more irregularly over the whole breadth of the depression. In adult shells the ribs are stretched in the depression. This often causes one or two ribs to run out where the depression reaches the margin. In about 20% of my specimens the ribs also alternate a little in the anterior part of the shell surface, but to a much slighter extent. The growth lines are everywhere visible on the shell surface. Where the ribs alternate, the growth-lines are visible as fine scratches on the ribs, or as fine bridges between the ribs. In well preserved shells mostly one or two concentric dark-coloured bands are present, often crossed by fine colour lines, radiating from the top. The colour pattern is better visible on a wet surface, especially the radiating lines.