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 tine, close, raised threads which cross the radials at right angles.
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.

As M. triggi:
Suborbicular, valves unequal, the right valve deeper than the left; auricles small and prickly, subequal, byssal gape minute, scarcely noticeable, pectinidial teeth small in the immature, obsolete in the adult; left valve pink, right valve white; sculpture of left valve, seven prominent distant compound ribs, having the intervals broader than the ribs; the right valve has, similarly, seven compound ribs whose intervals are narrower than the broad, flattened series whose edges fit into those of the left valve; the compound ribs of the left valve are composed of three ridges, rather sharp, the middle one the largest; the compound ribs of the right valve are made up of four to five low, flattened ribs, in early life scarcely separable but divergent with age; interstices in left valve have four to eight fine ridges, those of the right being deeper and with three or four ribs therein; very fine concentric threads overrun the whole of both valves in juvenile and unrubbed adult specimens; growth lines well marked. Holotype: length 50 mm, height 46 mm, section 25 mm. Great Aus¬tralian Bight, 80 miles west of Eucla, 100 fathoms. (Reg. No. D.13289, S.A. Mus.)
Loc.: Beachport to 120 miles west of Eucla, 40-140 fathoms. S.W.A, S. A..
Remarks: The Flindersian species is apparently of different shape, more numerously ribbed, bolder in sculpture than the Peronian Mesopeplum caroli Iredale, and recalls the Miocene fossil Chlamys polymorphoides Zittel and Chlamys incertus Tenison Woods 1867, though the fossils have marked concentric growth undulations. These species are probably generically the same as Mesopeplum triggi.

Type Material: Holotype - AMS.
Type Locality: Off Ball's Head, Sydney Harbour.
Original Description: "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 broad and high cant 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 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." (fide Hedley, 1901) Size: Normal adult size range is 40 to 50 mm.
Distribution: From southern Western Australia, all along southern Australia to southern Queensland. Habitat: Littoral to sublittoral zones, on muddy sand or sandy bottoms.
Remarks: For years M. fenestration was considered invalid and a synonym of M. caroli Iredale, 1929, due to the belief that the name had been previously used. Today, we know that Pecten fenestrates
Forbes, 1844 is actually a Parvamussium. This means that Hedley's taxon is not affected and is still valid. Some authors consider M. triggi (Cotton & Godfrey, 1938) to be a separate and valid species, however it is treated here as a synonym of M. fenestratum.
Relevant Literature: Hedley, 1901; Lamprell & Whitehead, 1992; Dijkstra, 1998; Beu & Darragh, 2001.