Downtonian Fossils from the Anglowelsh Area

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Abstract
Shell transversely oval to subrhomboidal, nearly twice as long as wide; hinge-line straight, less than length of shell; beaks small, low, contiguous, directed forwards, situated at about one-third (or less than one-third) length of shell; anterior end projecting, rather sharply rounded; ventral margin gently arched; posterior end rounded below, subtruncate above, meeting hinge-line at obtuse angle. Valves moderately convex, without distinct umbonal ridge, but with small subtriangular somewhat flattened or gently concave posterior area; anterior third or two-fifths of surface smooth, except for concentric striæ; middle three-fifths or half ornamented with 5 to 7 radial ribs somewhat obliquely directed backwards, of which the anterior 2 or 3 are broad, flattened, subequal, and separated by narrow grooves less than half their width; the posterior 3 or 4 ribs are narrower, closely placed, and less flattened, occasionally with small scabrosities on them; posterior area smooth, except for concentric striæ. A few submarginal concentric lamellose growth-ridges are occasionally present, and a fine granulation is visible, with a lens, over the whole surface. Anterior muscle-scar small, oval. Dentition unknown. Dimensions—Length 8–13 mm.; height 5–6 mm. Remarks.— This shell has been identified with Pleurophorus virelyi Barrois (Barrois & other, 1920, p. 145, pl. svi. fig. 10), of the Psammites de Liévin of l'Escarpelle, Liévin, but it seems to differ slighty in its more oval shape, in the greater number of radial ribs, and the fact that the last 3 or 4 are notably smaller than the anterior 3; the hinge-line also seems shorter and the

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