Ambonychia radiata

• Surface of the valves marked by strong, radiating ribs
• Ribs are crossed by fine, concentric, imbricating lines of growth
• Beaks acutely pointed and strongly incurved
• Anterior margin slightly concave
• Umbonal ridge evenly rounded

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Toni (1975):

• Material. – Five right valves, one left valve, two internal molds, two bivalve molds, and five incomplete molds. The specimens figured are 14303 and 16503.
• Locality and horizon. – South beach of Vestre Svartøya, Ringerike district; Holmenskjæret at Holmen, Tangen, Semsvannet, Jongskollen and Slependen in the Oslo-Asker district; Porsgrunn, Langesund-Skien district. Stage 5a. Gastropod Limestone, Ashgillian and Stage 5b, Calcareous Sandstone formation, Upper Ashgillian.
• Occurrence. – Originally described from the Lorraine Formation of New York. Recorded from the Maysville-Richmond in Quebec, Canada, also in Turin, Italy; and from Ohio, Indiana, Kentucky, Virginia and Tennessee, U.S.A. It is further recorded by Hind (1910) from the Drummuck Group, Scotland.
• Remarks. – This well known and widely distributed species is a common form in the Norwegian collection. The specimens at hand show the same characters as Hall’s species: Raised and incurved umbo, nearly obsolete anterior margin, obliquely truncate posterior side making an obtuse angle with the hinge line. The Norwegian species, however, seems to be larger than the American one. The specimen in Fig. 3, B has approximately fifty radial plications and is 29 mm long and 35 mm high.

Foerste (1924):

• Anterior margin slightly concave along upper four-sevenths of height of shell, owing to arching forward of the beaks and incurvature of anterior face of shell toward byssal opening. The upper part of the umbonal ridge never tends to be abruptly angular in a transverse direction as in B. richmondensis. Angle between the anterior face and the cardinal margin usually varies between 85 and almost 90degrees, but in some cases as low as 80 degrees. Length of the cardinal margin mostly about half the greatest length of the shell, from the beak toward the posterior part of the nearly evenly convex basal margin. In some, cardinal margin equals about three-fifths of greatest length of shell. Ratio of greatest width to greatest length, measured diagonally across the shell, from beak to posterior part of the basal margin, usually about 77 per cent, but may vary from 70 to 81 per cent. Convexity of single valve commonly not exceeding 6 mm in a shell having a maximum length of 36 mm. Shells exceeding 40 mm in length rare.
• The number of radiating plications varies considerably. Shells in which both cardinal margin and anterior face are well exposed frequently have 45, 46, or 47 radiating plications. Specimens occur which have fifty-five plications. In the specimens with forty-five plications, plications distinctly broader than the intervening grooves; in those with fifty-five, width of the plication exceeds that of the intervening grooves only slightly, if at all. Between both extremes intermediate forms occur in the same layers of rock.
• Wekwemikongsing Specimens. Numerous specimens of Byssonychia occur in the Lorraine exposure south of Clay cliffs , having about fifty radiating plicaitons. Umbonal ridge rather evenly rounded since the flattening of the anterior face of the shall, around the byssal opening, is very moderate. However, the general outline and convexity of these specimens resembles that of typical B. radiata sufficiently well to permit their reference to that species (No. 8517).
• Locality and Horizon. Originally described from the Lorraine formation of New York.
• Canadian Localities. In the province of Quebec it begins its range at the top of the Cryptolithus zone, Nicolet River section, St. Hugues, St. Hyacinthe, and Breault.
• In the Leptaena zone, in the Nicolet River section.
• In the Proetus zone, in the Nicolet River section, Chambly Canton, Vars, and Hawthore.
• Most widely distributed in the Pholadomorpha zone, occurring in the Nicolet River section, St. Hilaire, Weston, three-quarters of a mile south of Clay cliffs, and on the Bass Lake road south of Little Current.
• Similar forms, not known to be specifically the same, occur in the Waynesville in the Nicolet River section, at St. Hilaire, Huron river, Vars, Streetsville, 8 miles north of Meaford, and north of Manitowaning.
• In the Kagawong similar forms occur north of Manitowaning, west of the Indian village southwest of Little Current, and northeast of Gore Bay.
• In the Queenstown red clay shales, similar forms occur at several localities northwest of Meaford.
• Geographical Range. Outside of New York and the Canadian localities listed above, forms similar to B. radiata occur in the Maysville and Richmond of Ohio, Indiana, Kentucky, Tennessee, and Virginia. It is not certain that all of these forms are specifically identical with B. radiata of the Lorraine of New York, but at present all as a rule are assigned to this species.

Hall& Whitfield (1875):

• Shell small to medium sized, varying in outline from subquadrangular, with a rounded base, to acutely ovate, according to the degree of obliquity of the body of the shell to the direction of the hinge-line. Surface of the shell ventricose, and often subcarinate on the umbones and towards the beaks, gradually and somewhat regularly sloping to the basal margin, becoming attenuate and compressed toward the postero-cardinal region, and abruptly truncate and even impressed on the anterior side. Beaks acutely pointed, strongly incurved, terminal and projecting above the line of the hinge; posterior end at right angles to the hinge straight or rounded, or sometimes sloping obliquely backwards to the postero-basal margin; base sharply rounded. Anterior border of the valves excavated below the beaks, forming a rather large byssal opening, which is usually about half as wide as long when the valves are united.
• Surface of the valves marked by strong, radiating ribs, which are simple throughout, strongest on the body of the shell, and becoming finer on the postero-cardinal region. On the upper portion of the shell the ribs are flattened on the top, and often grooved in the center, giving them a strongly duplicate character, but becoming smooth below, the spaces between as narrow, or much narrower, than the width of the rib. The ribs are crossed by fine, concentric, imbricating lines of growth, which undulate as they cross the elevation.

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