|Basic info||Taxonomic history||Classification||Relationships|
|Morphology||Ecology and taphonomy||Age range and collections|
Strophomenata - Strophomenida - Hesperomenidae
It was assigned to Leptelloidinae by Williams (1962); to Leptellininae by Mitchell (1977); to Strophomenida by Sepkoski (2002); and to Hesperomenidae by Williams et al. (2000), Popov et al. (2000), Cocks (2005) and Benedetto (2012).
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|A. Williams 1962||Concavo-convex, elongately semi-oval to semicircular shells with a subdued carina in the pedicle valve and a complementary sharp postero-median indentation in the brachial valve; ornamentation finely multicostellate but with accentuated primary costae; ventral interarea long, apsaeline, delthyrium acutely triangular, pseudodeltidium possibly apical but mostly unknown and pedicle presumed functional, dorsal interarea shorter, anacline, chilidial arch complete and strong; shell substance pseudopunctate.
Teeth strong, disposed slightly lateral of the delthyrial edges and flanked ventro-medianly by a pair of medianly sloping ridges that are continuous along their ventral surfaces with the muscle-scar boundary and are the equivalent of the accessory teeth in other allied genera; muscle-field small, cordate and raised, with a low median ridge surmounted posteriorly by an elevated and elongately oval callosity that is also ankylosed to the delthyrial walls by a pair of acutely triangular sheets of secondary shell substance covering the postero-ventral portions of the accessory teeth; the resultant structure simulates a "pedicle chamber" and possibly acted in that capacity. Muscle-scar poorly differentiated, but the adductor bases were probably located postero-medianly on either side of the median ridge and under the free anterior margins of the pedicle chamber. Pallial sinus pattern obscure, except for slightly divergent impressions of the vascula media emanating from the antero-lateral edges of the muscle-field and the suggestion of saccate vascula genitalia.
Cardinal process complex, consisting of a massive boss, covered dorsally by the chilidium, undercut anteriorly and composed of a median ridge of fibrous calcite, flanked on either side by as many as 6 lateral ridges of cryptocrystalline calcite all embedded in secondary shell substance and forming a corrugated base for diductor attachment. The boss is contiguous laterally with a pair of curved plates to form a semicircular structure, convex posteriorly, which is homologous with the notothyrial platform and cardinal process of allied plectambonitaceids and is barely supported by a low, subdued, median ridge which passes anteriorly into the median septum of the lophophore platform. A pair of plate-like and divergent brachiophore bases occur laterally of the platform and are separated from it by deep excavations; the posterolateral faces of the bases define transversely oval sockets; the brachiophores are seen in serial sections to continue anteriorly in the line of their bases as a pair of curved plates. The lophophore platform is subquadrate in outline, strongly elevated, and slightly cleft antero-medianly where it joins a high, blade-like, median septum. The adductor scar is large and subflabellate; the pallial sinus pattern is probably lemniscate, comprising the vascula genitalia impressed laterally to the sockets, a very strong pair of divergent vascula myaria dividing the adductor scar, and a smaller pair of vascula media adjacent to the median septum.