Annals of the South African MuseumAnnale van die Suid-Afrikaanse Museum . The postero-dorsal margin is shallowly concave, and the respiratory margin subtruncate witha distinct posterior gape. The ligament pit is about 17 per cent of the shell length. The sunken escut-cheon is very broad, conspicuous, lanceolate, with fine, beaded, obliquelytransverse costellae that become obsolete before reaching the commissure. Inthe nepionic stages, the costellae of the escutcheon extend to the median longi-tudinal groove of the area, giving the inner area a beaded appearance for up to15 mm from the beak. A
Annals of the South African MuseumAnnale van die Suid-Afrikaanse Museum . The postero-dorsal margin is shallowly concave, and the respiratory margin subtruncate witha distinct posterior gape. The ligament pit is about 17 per cent of the shell length. The sunken escut-cheon is very broad, conspicuous, lanceolate, with fine, beaded, obliquelytransverse costellae that become obsolete before reaching the commissure. Inthe nepionic stages, the costellae of the escutcheon extend to the median longi-tudinal groove of the area, giving the inner area a beaded appearance for up to15 mm from the beak. A beaded marginal carina may extend for the same dis-tance, becoming rounded thereafter. The area is narrow, with a prominentlongitudinal groove that persists to maturity and is positioned slightly closer tothe escutcheon than the flank. In the nepionic stages it is ornamented by fine,oblique costellae that are more numerous than the flank costae, which they meetat the marginal carina in an anteriorly directed chevron. Beyond 15 mm fromthe umbo, the area is Fig. 20. Pterotrigonia rogersi (Kitchin), x 1. Left valve of a topotype in the South African Museum. In the nepionic stages the distinctly tuberculate flank costae are subconcen-tric. Later flank costae are strongly oblique but not sharply discrepant andextend to the anterior commissure. The anterior 12 or so costae are narrowerthan or as broad as the interspaces, finely but prominently tuberculate, andweakly curved until they reach the anterior face where they curve stronglyupwards to meet the anterior commissure almost at right angles. On the pos-terior half of the flanks the costae are straight, rigid, and inclined slightly to theposterior, with slightly narrower interspaces. 38 ANNALS OF THE SOUTH AFRICAN MUSEUM Discussion The side-by-side occurrence of two species of Pterotrigonia, Pterotrigonia(?Pterotrigonia) knighti and P. rogersi points to an early Neocomian radiation ofthe Pterotrigoniinae. Although P.
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Keywords: ., bookauthorsouthafr, bookcentury1800, bookdecade1890, bookyear1898
