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Strophomenata - Billingsellida - Polytoechiidae
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|G. A. Cooper 1956||Smaller than the preceding species ; wider than long and having a subquadrate to subrectangular outline. Valves subequal in depth, the brachial valve having a slightly greater depth. Sides and anterior margin rounded; anterior commissure broadly and gently uniplicate. Surface multicostellate ; costellae narrow and separated by spaces greater than the width of the costellae ; about 3 costellae in I mm. at the front margin.
Pedicle valve unevenly convex in lateral profile, the median portion somewhat narrowly rounded, while the anterior portion is flattened and bent toward the brachial valve and the posterior part is gently convex. Anterior profile forming a fairly strong arch with convex middle and long, strongly sloping sides. Umbonal region somewhat gently swollen ; median region tumid, sloping steeply in all directions except that toward the umbo ; interarea strongly apsacline, moderately long; foramen fairly large. Interior with small teeth having deep but small fossettes ; dental plates well developed, divergent. Delthyrial cavity moderately deep; diductor tracks wide and direct, separated by 2 parallel thin, low ridges which form an adductor track.
Brachial valve in lateral profile moderately but unevenly convex, the greater convexity in the posterior half and with the anterior half flattened. Anterior profile strongly domed, the top of the dome somewhat flattened and the sides with steep but short slopes. Median region inflated and with steep slopes laterally and anteriorly ; umbonal region somewhat inflated and marked by a short and shallow sulcus which disappears posterior to the middle ; anteromedian region somewhat swollen to obliterate the sulcus but not forming a distinct fold. Interior with broad brachiophores and deep sockets ; cardinal process with bulbous myophore ; median ridge almost obsolete, and confined to the umbonal region.
|A. Williams 1962||Subrectangular to subcircular, unequally biconvex Eremotoechia, commonly with a welldefined median ventral fold and dorsal sulcus in immature shells, changing to a subdued sulcus and fold respectively in adult forms; pedicle valve over one-quarter as deep as long and almost four-fifths as long as wide, with a strongly apsacline interarea one-quarter as long as the valve, brachial valve with rounded lateral areas and about one-fifth as deep as long; finely multicosteUate so that 5 costellae per millimetre commonly occur 5 mm. antero-medianly of the umbo; ventral muscle-scar slightly longer than wide, extending beyond narrowly divergent dental lamellae for about one-third the length of the valve; cardinal process bulbous, flanked by chilidial plates and ankylosed to socket plates lying along the hinge-line; adductor muscle-field vaguely impressed about a low, short, median ridge and divided by three pairs of divergent septa.|