Mesopeplum fenestratum (Hedley, 1901)
HEDLEY, C. 1901. Studies on Australian Mollusca. Part III. The Proceedings of the Linnean Society of New South Wales, 25 (4), no. 100: 721-732, pl. 48. [p. 730, pl. 48, figs. 17-19]
1901 Chlamys hedleyi Dautzenberg, 1901 [nomen novum pro Chlamys fenestrata Headley, 1901]
1901 Chlamys fenestrata Hedley, 1901
1929 Mesopeplum caroli Iredale, 1929
1938 Mesopeplum triggi Cotton & Godfrey, 1938
1901 Chlamys fenestrata Hedley, 1901
1929 Mesopeplum caroli Iredale, 1929
1938 Mesopeplum triggi Cotton & Godfrey, 1938
C. Hedley, 1901, plate 48.
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«Shell small, very solid, convex, almost equilateral and equivalve, inferior margin well rounded. Both auricles well developed, angular, with a few faint radii. Colour rose with irregular white splashes. Sculpture: the central third of the valve is occupied by a deep and broad furrow flanked by two broad and high ridges. On either side of these are two comparatively insignificant ridges with their complementary furrows. Each main radius and sulcus has three or four secondary radii, those in the hollows smaller and wider apart. The whole surface of the valve is microscopically ornamented by fine, close, raised threads which cross the radials at right angles. Fig. 19 shows a portion of the central furrow highly magnified.
The interior is stained deep rose. The adductor scar is deep. The impress of the main radii extend as furrows to the umbo, while the secondary radii merely denticulate the margin. The left valve develops a few pectinidial teeth. The chondrophore lies within the margin of the hinge plate. On either side of it and beneath the ligament is a narrow, vertically striated tract. Beneath this again is a slight cardinal rib. Height, 20 mm.; length, 20 mm. Hab.—The valve figured was dredged by Mr. J. Brazier in 18 fathoms off Ball's Head, Sydney Harbour. Another specimen was taken in 8 fathoms off Nelson's Head, Port Stephens, N.S.W. Type to be presented to the Australian Museum. The definition of Fischer's section Æquipecten appears to include this species, which more resembles Pecten fasciculatus, Hinds (Reeve, Conch. Icon, viii., PI. xxxv., f. 171), than any figure I can find.» CHARLES HEDLEY, 1901
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«Type data. Chlamys fenestrata Hedley: holotype (lv) AM C.008971 (Fig. 29C). Type locality: Australia, New South Wales, Sydney Harbour, off Ball’s Head, 18 fathoms [33 m].
Mesopeplum caroli Iredale: holotype (pr) AM C.170894. Type locality: Australia, southern New South Wales, off Montague Island, 55–60 fathoms [101–110 m]. Despite being allovated a catalogue number, the holotype is not recognisable in the Australian Museum. The illustrated specimen, AM C.347523 (Figs 29D–E, 31C–D), from southern New South Wales, off Jervis Bay, 37–124 m, closely resembles Iredale’s (1929: pl. 38, figs 6–8) illustration of the holotype, and is here designated the neotype of Mesopeplum caroli Iredale, 1929. Mesopeplum triggi Cotton & Godfrey: holotype (rv) SAM D13289, 3 paratypes (rv) SAM D15643. Type locality: Australia, SA, Great Australian Bight, 80 miles W of Eucla, 100 fathoms [183 m]. Comments on type data. Iredale’s (1929) assertion that Hedley’s species Chlamys fenestrata is based upon an adult specimen is incorrect. The holotype of Chlamys fenestrata is an immature specimen morphologically identical to adult specimens of Mesopeplum caroli.
Cotton & Godfrey (1938) introduced a species, Mesopeplum triggi, from the Great Australian Bight, which in their opinion is similar to the Miocene fossil species Pecten polymorphoides Zittel, 1865 (i.e., presumably intended for Mesopeplum minimum Beu & Darragh, 2001) and M. incertum (Tenison Woods, 1865), but which in our opinion is part of the variation of M. fenestratum. For comparison and discussion see Beu & Darragh (2001: 159; Beu et al., 2012: 34). For comparison with Mesopeplum convexum see below. (...)
Description. Shell up to c. 55 mm high, solid, moderately inflated, right valve slightly more convex than left, subcircular, weakly prosocoline, inequivalve, inequilateral, auricles almost equal in size and shape, moderately small; umbonal angle c. 90°; left valve whitish, cream or brownish, with magenta near the umbo of both valves and magenta maculations on plicae and costae, particularly towards dorsal margin, right valve otherwise uniform white.
Left valve sculptured with 3 prominent primary radial plicae and one weaker at each end of disc, with interspaces each broader than one plica; entire surface bearing secondary radial riblets. Auricles with 3–5 radial riblets. Surface bearing delicate, widely spaced, commarginal lamellae throughout, particularly preserved in radial interspaces, more closely spaced on auricles. Right valve with 4 broad, flatter radial plicae, more prominent on centre of disc than on ends, with narrower interspaces than on left valve. Secondary radial riblets and commarginal lamellae as on left valve, but radial riblets weaker on plicae. Anterior auricle with 4–6 riblets, weaker or lacking on posterior auricle. Dorsal margin straight. Byssal notch large in specimens up to 5 mm high, decreasing in size with growth, shallow by 15 mm high, very small in adults; functional ctenolium well-developed in juveniles, lacking in adults. Hinge teeth weak. Dimensions. Illustrated specimen: NSW, off Jervis Bay, 37–124 m (AM C.347523): rv: H 47.4, L 49.6 mm; lv: H 46.9, L 48.8 mm; D 20.4 mm. Hedley (1901: 730) stated the dimensions of the holotype of Chlamys fenestrata as “height 20, length 20 mm”.
Habitat. Living free (not byssally attached to substrates) on the continental shelf and in the upper bathyal zone (in 27–256 m) amongst rubble on soft sediment (sand and mud).
Distribution. All around southern Australia from North West Cape, Western Australia (Well & Bryce, 1985: 160), Great Australian Bight, South Australia, Tasmania, Victoria, New South Wales, northwards to southern Queensland. Maximum depth range 73–256 m (Cotton, 1961: 102). Present material alive at 27–165 m.
Remarks. Specimens from southwestern Australia (Mesopeplum triggi morph) are somewhat atypical in having coarser secondary radial costae, but intermediates are also observed and other characters are identical to those of M. fenestratum. As also stated by Beu & Darragh (2001: 173) this form is part of the variation of M. fenestratum. Iredale (1924: 194) correctly pointed out that Dautzenberg’s (1902: 347) replacement name Chlamys hedleyi for Chlamys fenestrata is unnecessary, as C. fenestrata Hedley is not a homonym of Pecten fenestratus Forbes, 1844.
Mesopeplum convexum from New Zealand differs from M. fenestratum in being larger, up to c. 70 mm high (M. fenestratum up to c. 55 mm), more variable in shape (more variable in prosocline shape and more variable in inflation), more variable in sculpture (number and prominence of plicae and prominence of secondary costae), much more variable in coloration, and in retaining a short functional ctenolium and moderately deep byssal notch in adults (see also Beu & Darragh, 2001: 173; Dijkstra & Marshall, 2008: 72, figs 61A–D, 62). Specimens of Mesopeplum convexum vary clinally, southern specimens reaching a larger size and tending to be more weakly inflated and more finely sculptured than northern ones. Mesopeplum fenestratum is extremely conservative in sculpture and coloration compared with M. convexum. Reeve’s (1853: pl. 22, fig. 84) illustration of Pecten roseopunctatus shows a species similar to Mesopeplum fenestratum, despite Cuming’s incorrect locality, “Moluccas”. The colour pattern also is similar to that of immature specimens of the common SE Australian form of M. fenestratum. Unfortunately, there is no type material of P. roseopunctatus in NHMUK. Dijkstra & Marshall (2008: 73) listed this name as a synonym of M. convexum, despite the unusual colour pattern. The specimen illustrated by Reeve (1853: pl. 22, fig. 84) has unusually coarse radial sculpture and a very prominent commarginal growth step, and closely resembles some of the more coarsely sculptured New Zealand specimens of M. convexum in these characters. We regard it as a synonym of M. convexum. The specimen of Mesopeplum convexum illustrated by Dijkstra & Marshall (2008: fig. 61C, D) from Stewart Island (NMNZ M155905) is here designated the neotype of Pecten roseopunctatus Reeve, 1853, to attach this name permanently to M. convexum rather than replace the name M. fenestratum.» DIJKSTRA, H. H. & A. G. BEU. 2018. Living scallops of Australia and adjacent waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae). Records of the Australian Museum, 70 (2): 113-330, figs. 1-102. [p. 176]
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Mesopeplum fenestratum (Hedley); H. H. Dijkstra & A. G. Beu, 2018, Living scallops of australia and adjacent waters, figures 29C–29E, 31C, 31D.
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