Late Ordovician (Caradoc – Ashgill)
Plectorthis is an extinct genus of stationary epifaunal suspension feeders
Identification in Hand Sample:
- Shells have distinct plano-convex contour, gently biconvex profile
- Anterior commissure faintly sulcate to uniplicate
- Ventral interarea short
- Ventral muscle field subcordate with linear adductor track contained by subcrescentic diductor scars
- Cardinal process a thick, rounded ridge
- A small-medium sized orthid brachiopod that has a gently biconvex profile. The rectangular to subcircular outline is truncated by a straight hinge line. Width is greater than length. Ribs radiate from the umbones toward the commissure. The ribs are coarser than those of Plaesiomys and Glyptorthis. Fairview formation.
Treatise on Invertebrate Paleontology, Part H, Vol. 3 (2000)::
- Transversely semioval, subequally gently biconvex with faintly sulcate to uniplicate anterior commissure, costate to coarsely costellate, filate; ventral interarea short, shorter dorsal interarea orthocline to faintly anacline; dental plates divergent, ventral muscle field subcordate with linear adductor track contained by subcrescentic diductor scars separated anteriorly by low median ridge; pedicle callist not reported; cardinal process a thick, rounded ridge and crenulated myophore; exopunctate with radialy distributed aditicules in some species.
Hall and Clark (1892):
Orthis: The distinguishing features of these shells are the plano-convex contour; the strong, sharp and comparatively few costae, rarely, if ever, bifurcating; the elevated and somewhat in curved cardinal area on the pedicle valve; the relatively slight development of the dental lamellae, which do not extend the entire length of the umbonal cavity. The cardnial process on the brachial valve is an elongate, vertical plate, extending from the apex the whole length of the delthyrium, thus longitudinaly dividing the deep deltidal cavity. It is usually simple, both on the outer edge and at its distal extremity. In this group of orthids, more frequently than elsewhere, we find a character rarely developed in any stage of growth, viz.: the existence of the transverse apical plate in the delthyrium of the pedicle-valve. This is probably homologous the the apical plate of the Spirifers, but is wholly distinct in origin from the covering of the delthyrium. The greatest development attained by this feature, in any of the numerous species of Orthis studied, is to be found in O. tricenaria of the Trenton and Hudson fauaas; it has also been observed in O. calligramma. Its appearance in this genus, and especially in the typical species of this genus, is interesting, but it can not be embraced in the diagnostic characters since its presence appears to be largely of a specific value, and the degree of its development dependant upon the stage of growth. The muscular scar of the pedicle valve of O. calligramma is a subelliptical area scarcely longer than the cardinal face, faintly impressed, and its components rarely distinguishable. The structure of the shell is compactly fiberous and impunctate. Specimens of Orthis calligramma from Gotland, show openings of oblique tubules on the external surface, always situated upon the keels of the costae. These are sparse and irregularly scattered, but of similar nature to those seen in the impunctate species, O. subquadrata, and the punctate species, O. subaequata, O. resupinata, and their allies.
Plectorthis: This is a persistent form, which in American faunas, so far as known, is limited to the Trenton and Hudson River formations. WHile it retains the strong external ribs of the typical Otrhis, these are not invariably simple; the cardinal area of the pedicle valve is comparatively low and the valves are subequally convex. In the interior the interior the character of the muscle scars, dental lamellae and cardinal process is essentially the same as in Group 1 (Orthis) and the minute structure of the shell appears to be in precise agreement with that of O. calligramma, thought no evidence of tubulose costae has been observed. In Orthis Jamesi, which is placed in this association, there is occasionally a deviation toward the resupinate contour exemplified in the groups IV and V.