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Published online by Cambridge University Press: 20 May 2016
Modern pinnoideans, or “pen shells,” are shallowly to deeply endobyssate, equivalved, edentulous, ham-shaped bivalves with a single, slightly submerged, opisthodetic ligament. Malacologists have generally regarded pinnids as closely related to the pterioid superfamily Pterioidea, especially its Paleozoic family Pterineidae (e.g., Zittel, 1927, p. 446; Thiele, 1935, p. 803; Pojeta, 1978; Waller, 1978). As early as 1890, Jackson suggested that pinnids evolved from the Silurian-Permian pterineid Leptodesma Hall, 1883. More recently, Pojeta (1978, p. 239) speculated that they evolved from the pterineid Pteronitella Billings, 1874 (a Silurian genus similar to Leptodesma) through the morphologically intermediate Devonian Palaeopinna Hall, 1883. The hinge and ligament of Palaeopinna remain unknown, so its family-level placement is uncertain (Newell and LaRocque, 1969, p. N301). However, Leptodesma and Pteronitella have hinge teeth, inequivalved shells, and a duplivincular ligament similar to other pterineids (Newell and LaRocque, 1969, p. N299–N301; Pojeta, 1978, p. 239; Pojeta and Runnegar, 1985, fig. 16D; Carter, 1990a, p. 205; 1990b, p. 334).