Argyrotheca cistellula (Forbes & Hanley, 1849)

Figures 38, 39

Terebratula cistellula Wood, 1840, p. 253 .

Megathyris cistellula – Forbes & Hanley (1849), p. 361 –363, pl. 57, fig. 9. Argiope cistellula – Davidson (1852b), p. 373.

Argiope (Cistella) cistellula – Gray (1853), p. 114.

Terebratula (Argiope) cistellula – Lovell (1861), p. 186.

Megathyris (Cistella) cistellula – Dall (1870), p. 146.

Cistella cistellula – Davidson (1887), p. 139 –140, pl. 22, figs. 1–4. Argyrotheca cistellula – Dall (1900), p. 44.

Argyrotheca lunifera Philippi – Helmcke (1938), p. 245 .

Description: Shell with transverse subangular outline and can have weakly indented front. Both dorsal and ventral valves with weak median furrow. Hinge line wide and straight. No ornamentation except for growth stops. Colour white, yellow or grey. Pedicle opening large and extending into dorsal valve. Deltidial plates not meeting. Shell matrix endopunctate. Brachial loop in dorsal valve long, simple, bilobed and anteromedially fixed. Triangular dorsal median septum sometimes with pillar-like anterior thickening. Median septum also developed in ventral valve, though smaller. Maximum shell length 3 mm.

Depth range: 2–255 m depth (Logan et al. 1997; Gaspard 2003). However, this species is rarely found deeper than 100 m.

Temperature range: 11–13˚C (Simon & Willems 1999).

Substrate: On bedrock, stones, sand or shells (Jeffreys 1863; this study).

Geography: Norway (Vestland, Møre and Romsdal, Trøndelag), East Shetland, Scotland, the Hebrides, Ireland, the English Channel, SE France, Spanish Bay of Biscay, Portugal, Sardinia, Aegean Sea, Sicily, Adriatic Sea, Greece, and the Azores (Jeffreys 1863; Friele 1873; Davidson 1887; Logan 1979; Brunton & Curry 1979; Besteiro & Urgorri 1986; Logan et al. 1997; Thomsen & Brattegard 1997; Simon & Willems 1999; Logan et al. 2007; Bitner & Gerovasileiou 2021; this study). From Norway, it has also been reported from Østfold, Nordland and Troms by Thomsen & Brattegard (1997), but despite searching, no specimens were encountered in the present study to support this. One empty shell was taken from Rogaland in SW Norway during the present study, suggesting its presence there.

Comment: This species was first named by Wood (1840); however, as pointed out by Jeffreys (1863), neither a description nor an illustration was provided with the name. Due to the lack of a description, the naming is not a true one in the sense of the International Code of Zoological Nomenclature (Article 12); therefore, it is here assigned to Forbes & Hanley (1849), who provided the first description.

Argyrotheca cistellula is uncommon in samples from Norway but seems to prefer shallow waters. Although the species is undoubtedly uncommon, its small shell size additionally makes it easy to overlook.