Semipallium hallae (Cotton, 1960)

Dijkstra, Henk H. & Beu, Alan G., 2018, Living Scallops of Australia and Adjacent Waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae), Records of the Australian Museum (Rec. Aust. Mus.) 70 (2), pp. 113-330 : 267-268

Semipallium hallae (Cotton, 1960)

Figs 81, 82G–H, 84A–C

Notochlamys hallae Cotton, 1960 : [1], 2 unnumbered figs; Cotton, 1961: 99, fig. 85; Rombouts, 1991: 47.

Semipallium hallae (Cotton) .–Lamprell & Whitehead, 1992: [28], pl. 12, fig. 71 (holotype); Raines & Poppe, 2006: 242–243, lower figs; pl. 185, fig. 3.

Type data. Holotype (pr) SAM D16541. Type locality: South Australia, Largs Bay , Semaphore Beach.

Comments on type data. The listed and figured articulated paratype (H 50 mm, W 44 mm) is not in the SAM collection, but is possibly in the Hall collection.

Additional material examined. — AUSTRALIA: SOUTHAUSTRALIA: Yorke Peninsula , Edithburgh Jetty, alive, 4–5 m (1 pr, SAM D19012) ; Adelaide , West Beach, c. 34°56'S 138°36'E, dead, beach (1 v, WAM S12924 ) ; Gulf of St Vincent , Port Adelaide, Outer Harbour, alive, 5 m (3 pr, SAM D19013) ; Gulf of St Vincent , Henley Beach, dead, beach (2 v, SAM D19014) ; offshore from Glenelg , 34°59'S 138°31'E, low limestone reef, alive (1 pr, ZMA Moll. 146627) .

Description. Shell up to c. 55 mm high, weakly inflated, almost equally convex, elongate, somewhat higher than wide, slightly inequivalve and inequilateral, anterior auricles larger and longer than posterior, umbonal angle c. 95°; most specimens purplish, umbonal area paler; right valve paler; interior deep purplish.

Both valves sculptured with 8–9 evenly spaced, weakly defined primary radial plicae (narrower on left valve, wider on right valve), less prominent towards anterior and posterior ends of disc; secondary radial riblets present on plicae (c. 3 on left valve, 4 on right valve) and in radial interspaces. Interstitial shagreen microsculpture throughout. Anterior auricles bearing c. 7–10 radial riblets, posterior bearing 4–6. Dorsal margin straight. Byssal notch moderately deep, byssal fasciole rather broad. Functional ctenolium weakly developed, with c. 6 teeth, or almost obsolete in mature stage. Internal corrugations well-defined, corresponding to external radial plicae; edges sharply defined, carinate around ventral margin. Hinge with prominent resilial and dorsal teeth.

Dimensions. Illustrated specimen: SA, offshore from Glenelg, 34.9667°S 138.5167°E, low limestone reef (ZMA Moll.143774): H 48.6, L 49.4, D 15.2 mm.

Habitat. Living in the littoral zone in crevices of limestone ledges at silty reef edges, hidden in shallow areas amongst rubble, and byssally attached to hard substrates.

Distribution. This species has a very limited distribution in South Australia, in particular in the Gulf of St Vincent. Present specimens alive at 4– 5 m.

Remarks. Cotton (1960) placed this little-known species in Notochlamys Cotton, 1930 , based mainly on the shagreen microsculpture. However, Semipallium hallae differs from Notochlamys species in several characters: more elongate in shape ( Notochlamys more nearly circular), less clearly defined primary radial plicae (well-defined in Notochlamys ), and great reduction in the size of the posterior auricles (in Notochlamys the anterior and posterior auricles are of almost identical size). Semipallium hallae is more appropriately placed in Semipallium .

Beu & Darragh (2001: 76) synonymized Semipallium hallae with S. aktinos ( Petterd, 1886) with a query, having seen no material of this very restricted species. In our opinion the two are distinct and differ in the larger size, wider shape, more prominent radial plicae, and more uniform purplish colour of S. hallae than of S. aktinos .