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Strophomenata - Strophomenida - Plectambonitidae
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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 of about medium size for the genus, nearly twice as wide as long at the hinge and with the hinge forming the widest part. Cardinal extremitites acute to alate. Front margin broadly rounded to truncate. Surface marked by costellae of unequal size; 2 to 4 of the smaller costellae occupy the space between thelarger ones ; about 16 costellae occupy a space of 5 mm. at the front of a valve about 6 mm. long.
Pedicle valve flat to slightly concave in lateral profile; beak slightly projecting posterior to the posterior margin ; umbo narrow, slightly swollen and produced forward as a narrow, low fold which may or may not disappear at about the middle of the valve. Anteromedian portion anteriorly depressed to form a shallow, wide sulcus. Area bounding sulcus slightly folded; lateral areas nearly flat to concave. Interarea moderately long, apsacline.
Brachial valve with sulcate umbo ; sulcus extending nearly to middle of valve ; anteromedian half of valve undulated into a broad fold; lateral areas slightly convex and with steepening slopes toward the margins. Area in front of cardinal extremities sulcate. Interarea moderately long but shorter than the interarea of the pedicle valve, hypercline.
Interior of pedicle valve well thickened ; delthyrial cavity small, choked with excess shell at the apex ; ridges between pallial impressions thick ; surface covered by small granules.
Interior of brachial valve with paired septa extending nearly to the front margin but with the crests of the septa at about the middle of the valve ; adductor field small, surrounded posterolaterally by narrow ridges. Radial thickenings between pallial impressions moderately strong.