(Turton, 1819)
Description (shell):
Shell thin and brittle, globular, each valve strongly convex, rounded posteriorly, with a right-angled indentation anteriorly, exposing a pedal gape. Shell constructed of several distinct elements: a triangular anterior lobe dorsal to the gape, with regular horizontal ridges; an elongate anterior disc extending from the umbo to the ventral margin, forming the posterior edge of the gape, with regular, oblique ridges; the median disc, a concave band with raised margins, sculptured with very fine concentric lines; the posterior disc, comprising more than half the shell, with fine concentric lines (X. dorsalis-detail). Umbonal reflection small, closely applied to umbo posteriorly, obscured by a bipartite mesoplax. On inner surface anterior edge of median disc developed as a prominent ridge. No apophysises, although left valve has a prominent chondrophore (X. dorsalis-drawing).
Size:
Up to 1.3 cm in length.
Colour:
Glossy and almost transparent. Inner surfaces white.
Animal:
The globular animal has long and slender siphons, connected throughout the greater part of their length and marked by crested ridges. The foot is thick and capable of considerable extension and functions as a boring instrument.
Habitat:
Boring into wood.
Distribution:
Distributed from Iceland and Norway south to the Mediterranean, not in the southern North Sea (Distr. X. dorsalis).