- Genus of Plectorthidae with large, subquadrate shells characterized by obcordate ventral muscle scars with thickened margins and adductor scars tracks raised on a double median ridge. Pallial markings rare to absent. Valves are unequally biconvex to convexoconcave; multicostellate with aditicules; sulcus median, typically wide; cardinal extremities are rounded to alate; large ventral interarea, catacline to apsacline; dorsal cardinal area long, apsacline; brachiophore plates convergent; dorsal muscle scars quadripartite, weakly impressed, with posterior adductor scars larger than anterior. Juvenile or smaller shells may be semielliptical. Emended from Williams and Harper (2000).
Treatise on Invertebrate Paleontology, Part H, Vol. 3 (2000):
- Mainly large, subquadrate with variable angled cardinal extremities, convexoconcave, uniplicate, multicostellate with aditicules, filate; both interareas relatively long, dorsal apsacline; dental plates divergent, ventral muscle scar subcordate, bounded by ridge, with elongately oval adductor scars impressed on double ridge, not enclosed by diductor tracks; adult cardinal process thick ridge with compressed myophore, brachiophores plates convergent, recessive; posterior pair of quadripartite adductor muscle scars larger than anterior pair.
Fossils of Ohio (1996):
- A large, subquadrate orthid brachiopod that is biconvex in profile. The pedicle valve may be flatter than the brachial valve. The width is markedly greater than the length, and the hinge line is straight. Many fine ribs radiate from the umbones. The beak of the brachial valve may be incurved. The brachial valve has a well-developed fold at the anterior commissure, and the pedicle valve has a corresponding sulcus. The pedicle valve has a raised beak with a triangular interarea and a triangular delthyrium. Muscle scars on the interior of the pedicle valve have three lobes. The two larger outer muscle scars are from the diductor muscles, and the central scar is from the adductor muscles. Cincinnatian Series.
Hall and Clarke (1892)
- This division is distinguished both by its external and internal characters; the pedicle valve has a well developed, often much elevated cardinal area and a long, straight hinge-line; its surface is depressed convex, always less convex than the opposite valve which is frequently gibbous or inflated. The surfaces is covered with a great number of fine, rounded, closely crowded plications which increase rapidly by intercalation, and are crossed by lamellose growth-lines, and fine concentric striae. On the interior of the pedicle valve the teeth are large and supported by thick lamellae which are continued as a strong ridge around a short, obcordate muscular area. This area is medially divided by a prominent ridge upon the summit of which lies the linear scar of the adductors. The flabellate lateral impressions are sometimes divisible into their two components, diductors and adjustors, and in old individuals the impression of the pedicle-muscle is often distinct.
In the brachial valve the dental sockets are narrow and are enclosed beneath and on the inner side vy the strong crural plates. The cardinal process is elongate and simple, sometimes thickened, often crenulate, but not lobed at its posterior extremity. This process unties with the inner bases of the crural plates and is produced forward as a median ridge dividing the four muscular scars, which are distinctly developed only in old shells.
The shell-structure is fibrous-impunctate, and the plications of the surface sometimes tubulose.
Shells of this type of structure are abundant in the Trenton and Hudson faunas and extend upward into the Clinton group but are not at present known in any later period.