Megerlia King 1850

The genus Megerlia King, 1850

The main question of this paper is “to be or not to be Megerlia ”. The type species of this genus is Megerlia truncata (Linné, 1767). The original material described by Linné as Anomia truncata apparently consists of two dried specimens (as indicated in the Catalogue of Type specimens of the University of Uppsala: n°1907a and n°1907b) and should be Norwegian specimens (“ Habitat in Pelago Norvegico Supra corallia ” Linné, 1767; p. 1152, n° 229). However, the presence of this species off Norway was not mentioned by Logan (2007) or Hiller et al. (2008). The species was not found in northerly latitudes such as the Irish coasts by Massy (1925), Norway by Wesenberg-Lund (1939), the North Sea (Cori, 1933) and the Faroe Islands (Thomsen, 2001). In the 19th century Megerlia species were not recognized along the British coasts (Forbes & Hanley 1850, 1853). The original Linnaean specimens may have come from another area in the eastern Atlantic or even from the Mediterranean Sea. The current genus description provided by Lee & MacKinnon (2006, p. H2245) stresses the large foramen which is often abraded (considered submesothyrid to amphithyrid), the disjunct deltidial plates and the tuberculate internal valve floors. No cardinal process is developed. Crural bases are present and attached to the inner sides of socket ridges. Descending branches join the cardinalia with the posterior part of the ring of the loop. The brachidium is typically bifurcate (see MacKinnon & Lee, 2006, fig.1312) and a complete ring is built; this is unique among Kraussinoidea.

SEM observations show in detail the development in M. truncata and interpretation of the earliest growth stages is here discussed again. The smallest specimen collected from Anzio has a width of 1.4 mm (Pl.1, Figs. 1a, 1b). This specimen is slightly biconvex. The tip of the beak is abraded but the submesothyrid foramen is still intact on its dorsal side. The dorsal valve is smooth and the ventral valve presents some rare tubercles near the anterior commissure. The second specimen (Pl. 1, Figs. 2a, 2b) has a width of 1.73 mm. The shell becomes ventribiconvex with a smooth dorsal valve whereas the external surface of the ventral valve presents a radial series of tubercles. Noteworthy is that the outer shell surface between the radial ornamentation is smooth. The beak is more abraded and the dorsal valve is affected as the foramen seems amphithyrid. This gradual change is quite continuous during growth as seen on Pl. 1, Figs. 3–7. It appears that the foramen is more and more abraded due to the shortness of the pedicle and the high energy environment where M. truncata is generally living. Another development is the appearance of radial costae with tubercles on the external side of the dorsal valve. This tuberculation becomes more important during growth (Pl. 2, Figs. 1–3) although the ventral tubercles always remain more developed in size. An underestimated character in M. truncata is the absence, at all growth stages, of secondary tubercles, spines or scales in the “intercostal” zones. In these zones the shell remains smooth and only concentric growth lines are perceptible (Pls.1–5). This is not the case in M. echinata (Pl. 10, Figs. 1, 2a–d) as strong spines cover the whole external surface of the ventral valve (see also Fischer & Oehlert, 1891, pl.7, figs 13a and b).

Examination of many specimens of M. truncata from the whole Mediterranean Sea also clearly shows that both valves are provided with radial tuberculation, the ventral tuberculation always being stronger than the dorsal one. Specimens with abraded dorsal tubercles are found but an entirely smooth dorsal valve without any tuberculation has not been observed. This feature is important for distinguishing M. truncata from M. echinata (see below).