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Rhynchonellata - Orthida - Plaesiomyidae
If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
|N. Hiller 1980||Dorsi-biconvex to convexo-concave shells of transversely sub-elliptical outline, maximum width anterior to hinge-line. Pedicle valve almost three-quarters as long as wide and one-sixth as deep as long, maximum depth close to umbo. Cardinal angles obtuse. Interarea about one-fifth as long as wide and about one-fifth as long as valve; flat, varying from apsacline to catacline; delthyrium open. Lateral profile varying from very gently convex to slightly concave; anterior profile slightly convex to flat. Brachial valve evenly convex in both profiles with faint sulcus modifying anterior profile in 3 out of 8 specimens. Depth over one-quarter of valve length on average but ranging from one-sixth to one-half. Interarea very short, less than one-tenth of valve length, curved orthocline to slightly apsacline. Ornament of subangular to rounded costae and costellae with about 17 primary costae on pedicle valve increasing to 28-34 ribs by 10 mm stage.
Ventral interior with teeth supported by receding dental plates which continue anteriorly to form low ridge bounding sub-pentagonal muscle field. This is almost as long as wide and extends for just over one-third of valve length. Pedicle callist small, medianly situated adductor scars small, elongately oval and almost enclosed by triangular diductor lobes; lateral diductor lobes well developed.
Dorsal interior with simple brachiophores diverging at about 90 and extending for about one-sixth of valve length, and supported only by secondary shell substance. Notothyrial platform moderately thick with high cardinal process differentiated into narrow shaft and grooved myophore. Low ridge extends anteriorly from platform to bisect slightly impressed adductor field which is just over two-thirds as long as wide. Suboval posterior scars larger than subquadrate anterior pair.