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Rhynchonellata - Orthida - Harknessellidae
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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.
|G. A. Cooper 1956||Shell large, rectangular in outline with the width slightly greater than the length. Cardinal extremities varying from acute to obtuse; lateral margins slightly concave to gently rounded ; anterolateral extremities somewhat narrowly rounded; anterior margin truncated. Anterior commissure rectimarginate to sulcate. Surface fascicostellate with 5 or 6 well-defined fascicles on each side of the fold. Fold consisting of 3 fascicles, a median one and one on each side of it. Costellae appearing in 4 generations by bifurcation and intercalation; 7 to 10 costellae may be counted in 5 mm. at the front margin.
Pedicle valve gently convex in lateral profile with the greatest convexity in the posterior half. Fold originating at the beak where it is very narrowly rounded and generally smooth, widening gradually anteriorly and heightening to about the middle from which it gradually declines in height to the front margin. Fold scarcely distinguishable at the front margin of some specimens except for the bundling of costellae. Lateral slopes of fold steep ; flanks bounding fold flattened or gently concave, becoming gently convex in a posterolateral direction where the areas just anterior to the cardinal extremities are gently convex. Slopes to cardinal extremities gentle. Interarea short, apsacline.
Brachial valve very gently convex with the greatest convexity in the posterior half ; about as deep as the pedicle valve. Sulcus narrow and deep in the posterior third, becoming wider and shallower anteriorly to the front margin where it occupies about one-quarter the valve width. Folds bounding sulcus strongest in the posterior, becoming flatter anteriorly and almost disappearing along the anterolateral margin. Slopes to lateral margins gentle.
Pedicle interior having small muscle field with width about equal to length ; teeth small ; dental plates short, stout. Median ridge inside the brachial valve heightened by the exterior sulcation which is most prominent at the posterior. Notothyrial cavity much restricted by secondary shell built on the inner sides of the short brachiophores ; cardinal process short and stout.
|A. Williams 1962||Large, subquadrate, unequally biconvex Reuschella with slightly rounded cardinal angles, pedicle valve almost three-quarters as long as wide, sharply and narrowly carinate medianly with gently convex flanks almost one-third as deep as long; brachial valve deeply sulcate with convexly rounded shoulders almost orte-fifth as deep as long; radial ornamentation fascicostellate with well-differentiated sectors of angular costellae, sectors I and II occupying the fold and sulcus, the latter including externally branching costellae on the brachial valve; ventral muscle-scar subcordate, slightly wider than long and extending anterior to the divergent, short, dental lamellae for over one-quarter of the length of the pedicle valve; cardinal process consisting of a massive, conical, highly crenulated myophore and short shaft, adductor pits shallowly impressed into the secondarily thickened brachiophore supporting plates just lateral to their convergence with a low median ridge that merges anteriorly with a sharp elevation representing the internal surface of the dorsal sulcus.|