Unionites Wissmann, 1841

Foster, William J., Danise, Silvia & Twitchett, Richard J., 2017, A silicified Early Triassic marine assemblage from Svalbard, Journal of Systematic Palaeontology 15 (10), pp. 851-877 : 862

Genus Unionites Wissmann, 1841

Type species. Unionites muensteri Wissmann, 1841 ; Carnian , Late Triassic; South Tyrol, Italy .

Diagnosis. The generic diagnosis follows Geyer et al. (2005). Shell suboval to elongate-elliptical or rectangular to trapeziform; equivalve, inequilateral. Umbo prosogyrate. Shell surface smooth except for growth rugae, or with rather fine commarginal riblets and, rarely, a faint radial ornamentation. Anterior hinge margin of right valve projects beyond the plane of commissure, fitting into a recess in the opposite valve. Small subumbonal groove limits this shell projection posteriorly. Posterior lateral tooth of right valve generally distinct, engaging below posterior dorsal margin of left valve. Hinge of left valve with anterior platform that bears depression for corresponding anterior hinge margin of right valve, followed posteriorly by small, tuberculiform subumbonal tooth. Narrow groove extends parallel to margin at distal part of anterior hinge margin and corresponds to anterior lateral tooth of right valve. Lower part of posterior dorsal margin slightly projects beyond plane of commissure, fitting above posterior lateral tooth of right valve. Ligament fixed on nymph, which extends about half the length of the posterior dorsal margin.