. Bulletins of American paleontology. Trigoniid Bivalves of Argentina: Leanza 17 Costa: A moderately prominent elevated ornamental feature of the outer shell surface that may be oriented radially, obliquely or concentrically. Same as "rib" in previous literature. Costella: A small and narrow, elevated ornamental feature of the shell, usually present in the area and/ or escutcheon, which is normally radial, as in Tri- gonia. but may be oblique, as in Apiotrigonia. or concentric, as in Frenguelliella. Same as "riblet" in previous literature. Dorsal margin: The margin of the s
. Bulletins of American paleontology. Trigoniid Bivalves of Argentina: Leanza 17 Costa: A moderately prominent elevated ornamental feature of the outer shell surface that may be oriented radially, obliquely or concentrically. Same as "rib" in previous literature. Costella: A small and narrow, elevated ornamental feature of the shell, usually present in the area and/ or escutcheon, which is normally radial, as in Tri- gonia. but may be oblique, as in Apiotrigonia. or concentric, as in Frenguelliella. Same as "riblet" in previous literature. Dorsal margin: The margin of the shell extending from the beaks to the dorsoposterior junction. Dorsoposterior junction: The normally angular, pre- dominantly obtuse, intersection between the dorsal and posterior margins, forming the dorsoposterior angle. Escutcheon: A dorsal surface on either side of the lig- amental groove extending posteriorly from the beaks. The escutcheon is commonly bounded by the es- cutcheon carina in each valve. The shape may be highly excavated to strongly convex, as in Trigonia aliexpandita. The escutcheon surface may be smooth, or usually ornamented by radial, oblique, or con- centric growth lines or costellae, but exceptionally it can bear transverse costae, as in Neuquenitrigonia. Escutcheon carina: A narrow elevated ridge or keel extending posteriorly from the beak along the dorso- lateral margins of the escutcheon and forming a dis- crete border. Flank: The entire valve surface between the anterior and ventral margins and the marginal carina. The term flank is equivalent to "disk" of the bivalve literature, but is more used by trigoniid workers. The ornamentation of this surface is important in the distinction of different taxa within the family. Intercostal space: Concave depression between adja- cent costae. Ligamental fossette: A narrow depressed lunate plat- form extending posteriorly from the beak along the dorsal margin, within the escutcheon, and serving for att
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