Megathiris detruncata (Gmelin, 1791)

Figures 36, 37

Anomia decollata Chemnitz, 1785, p. 96, pl. 78, fig. 705.

Terebratula ungula Philipsson in Retzius (1788), p. 15.

Terebratula detruncata – Bruguière (1789), pl. 213, fig. 10.

Anomia detruncata Gmelin, 1791, p. 3347

Terebratula urna antiqua Risso, 1826, p. 388, pl. 12, fig. 177. Terebratula cardita Risso, 1826, p. 389, pl. 12, fig. 180.

Terebratula aperta Blainville, 1828, p. 144 .

Terebratula decollata – Deshayes (1832), p. 1029.

Terebratula dimidiata Scacchi, (1833), p. 17.

Argiope detruncata – Deslongchamps (1842), p. ix.

Orthis detruncata – Philippi (1844), p. 69.

Megathiris detruncata – d’Orbigny (1847b), p. 253, pl. 7, figs. 26–27.

Terebratula pectiniformis Costa, 1844, p. 39 –40, pl. 1, fig. 6.

Argiope decollata – Gray (1853), p. 113.

Megathyris (Megathyris) decollata – Dall (1870), p. 145.

Megathyris detruncata – Dall (1920), p. 330 –331.

Description: Shell highly ventribiconvex. Outline subtriangular to semicircular. Ventral umbo high or long. Straight hinge line. Anterior commissure rectimarginate. Ventral umbo high or long and with high apsacline to almost catacline interarea (plane area between hinge line and umbo) bisected by long V- or U-shaped pedicle opening also extending slightly into dorsal valve. Deltidial plates narrow and triangular. Pedicle collar strongly raised but relatively short. Ornamentation of 7–18 broad, radiating ribs, and shell surface marked by growth lines. Ribs fading away distally on largest specimens. Shell matrix endopunctate. Colour light honey or greyish. Dental plates lacking. Ventral valve floor with narrow and low median septum or ridge. Long brachial loop reaching 2/3 valve length and attached to median and two or (rarely) four lateral ridges or septa. Dorsal median septum or ridge reaching close to valve front. Maximum shell size 6.7 mm long.

Depth range: Minimum range is 5–1940 m depth (Gaspard 2003; Logan 2007). The species is rarely found deeper than 500 m.

Temperature range: 11.5–25˚C (Jeffreys 1878; Brand et al. 2003).

Salinity range: 37–38 (Brand et al. 2003).

Geography: English Channel, Southernmost Celtic Sea, Bay of Biscay, Portugal, the Mediterranean, Madeira, the Canary Islands and the Cape Verde Islands (Jeffreys 1878; Logan 1979; Logan et al. 2007; Evangelisti et al. 2011).

Remarks: Bruguière (1789) did not provide a description with the name, which is why the authorship of the name was transferred to Gmelin (1791). The year of authorship is 1791, not 1790 as often cited in the literature.