Bucardium

Perna, Rafael La, 2016, Bucardium grateloupianum n. sp. from the Lower Miocene of Aquitaine with taxonomic comments on some fossil species from Europe (Bivalvia, Cardiidae), Zootaxa 4178 (4), pp. 568-582 : 569

Bucardium and its type species

Bucardium was introduced by Gray (1853), with the following diagnosis: "Shell subglobose, costate, hinder gape narrow and strongly toothed on the hinder edge". He included three species, Cardium ringens Bruguière, 1789 , C. procerum G. B. Sowerby, 1833 , and C. unedo Linnaeus, 1758 however, the type species C. ringens was subsequently designated by Vest (1875). It also became the type species of Ringicardium , proposed by Fischer (1887: 1037) as a section (subgenus) of Cardium .

Keen (1969) considered Bucardium as a subgenus of Cardium , with a distribution from Lower Miocene to Recent, in southern France and West Africa, and the following diagnosis: "Posterior ribs strongly digitate, no posterior gape".

Popov (1977), in a systematic arrangement mainly based on shell structure, considered Bucardium a distinct genus, subdivided into three genera, in addition to the nominotypical one: Vepricardium Iredale, 1829 , Europicardium Popov, 1977 , and, dubitatively, the extinct Agnocardia Stewart, 1930 . Cardium ringens remained the only species assigned to B. ( Bucardium ). Popov's classification has found no general agreement ( Keen 1980; Schneider 2002; ter Poorten 2005; La Perna 2016), but both Vepricardium and Bucardium are currently included in the subfamily Cardiinae ( Schneider 2002) whereas Europicardium is sister taxon of the clade Cardium + Bucardium ( Herrera e t al. 2015).

Bucardium ringens ( Fig. 1 ) has an almost orbicular shell shape, slightly higher than large, sub-equilateral, with a long, barely convex, gently sloping antero-dorsal margin, and a very short postero-dorsal one. The maximum size is about 50 mm in antero-posterior length (pers. comm., ter Poorten 2016). The sculpture consists of radial ribs, much finer anteriorly, wider and stronger on the remaining shell surface. Along the posterior margin, the radial ribs give origin to short, strong digitations ( Fig. 1 C) that are almost interlocking when the shell is closed and occupying most of the posterior shell gape ( Fig. 1 E).

The fine anterior ribs are symmetrically triangular in cross section, then they become more and more asymmetrical (saw-tooth shaped) toward the posterior margin, with a wider, slightly convex anterior slope and a narrower, stepped posterior one ( Fig. 1 A, C). Near the posterior margin, the ribs are distinctly ridged and strongly asymmetrical, forming the digitate margin with their projections ( Fig. 1 C). The radial interspaces are narrow, flat and smooth, except for the posterior area, where they are narrow and deep. Well distinct crescent-shaped growth striae are present on the posterior ribs ( Fig. 1 C). No sculpture, except for growth striae, occurs on the ribs. Details on the hinge of B. ringens were reported and commented by Schneider (2002: fig. 12B). In particular, he remarked the occurrence of striae on the lunular flap, a character shared with Cardium and some other Cardiinae genera. Schneider's illustration is not particularly clear, but the ridges on the lunular flap consist of a short, slightly oblique and somewhat irregular crenulation of the dorsal margin, more or less extending onto the lunular flap ( Fig. 1 G, H).

The species has a wide distribution in West Africa, from Mauritania to Angola (about 20°N–20°S), including the Cape Verde Islands and São Tomé and Príncipe, in 15–100 m ( Fischer-Piette 1977; Voskuil & Onverwagt 1989).

Figures

FIGURE 1. Bucardium ringens (Bruguière, 1789). A – F. Casamance, Senegal, depth 10 – 20 m (ter Poorten coll., TP 634), photo J. J. ter Poorten, with permission (A – C, RV; D, LV). G, H. Hann Bay, Senegal, RV (La Perna coll.).