Mesopeplum contrainflatum Beu & Darragh, 2001
BEU, A. G. & T. A. DARRAGH. 2001. Revision of southern Australian Cenozoic fossil Pectinidae (Mollusca: Bivalvia). Proceedings of the Royal Society of Victoria, 113: 1-205, figs. 1-67 [p. 163, figs. 58A-D]
2001 Mesopeplum (?) contrainflatum Beu & Darragh, 2001
A. G. Beu & T. A. Darragh, 2001, figure 58.
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«Description. Moderately large for genus (to 52 mm high and 55 mm long), markedly longer than high, strongly prosocline, posterodorsal margin up to 1.5 times length of anterodorsal margin; LV moderately to very strongly inflated and RV weakly to very weakly inflated; umbonal angle c. 110°. Plicae very low and weak, c. 3-5 discernible over proximal 15 mm of LV, fading out gradually below that, scarcely visible at all on RV. Radial sculpture of 20-22 low, square-edged primary radial costae over proximal 15-25 mm of disc, gradually widening and subdividing irregularly more distally to form many irregular, low, wide, indistinct costae, on some specimens grouped regularly into three narrow costellae over at least part of disc, in others arranged as roughly alternating wider and narrower costae over at least part of disc, random in others, tending to remain higher and less frequently subdivided on RV than on LV; low, narrow and widely spaced on auricles, very weak on posterior auricle of some specimens. Whole surface crossed by many, thin, well elevated, relatively widely spaced commarginal lamellae, closely spaced and forming dominant sculpture on auricles. RV anterior auricle (complete on only one specimen) moderately long, with wide, shallow, functional byssal notch, ctenolial teeth raised on a very narrow ridge and upper margin of notch edged with small, sharp points at ends of radial costae of auricle as in M. meringae. LV anterior auricle moderately long, triangular, with anterior margin inclined strongly anteriorly. Posterior auricles small, short, with weakly concave posterior margins. Hinge visible in only one RV; with very low dorsal teeth but moderately prominent resilial teeth beside dorsal half of resilium. Interior almost smooth, with very weak, rudimentary rib carinae around outermost ventral margin.
Dimensions.
Holotype NMV P302093 H 52.2 L 54.8 LV Paratype, NMV P302092 48.4 48.3 RV Paratype, AGSO CPC F888 51.7 53.7 2 valves Type material. Holotype, NMV P302093, PL3118, lower Jemmys Point Formation (Cheltenhamian), floor and sides of disused tramway cutting, north of Scrivenors Road, west side of Mississippi Creek, near Lakes Entrance, Gippsland, Victoria, grid ref. Colquhoun 836116, coll. F. A. Cudmore, 1936, 1939; one paratype, NMV P302092, RV, all data as above, coll. A. G. Beu & T. A. Darragh, 19.xi.1993; one paratype, NMV P302094, an incomplete RV, all data as for holotype; one paratype, an articulated pair with incomplete auricles, AGSO CPC F888, from ‘Tram track, Mississippi Creek, Lakes Entrance, Victoria’, ie., from the type locality, but accompanied by one valve of M. divergens sp. nov., which is probably mislocalised as we have seen no other material of M. divergens from the tramway cutting at Mississippi Creek, and, indeed, its apparent successor, M. meringae (Tate), is known by a few specimens from the Mississippi Creek cutting.
Other material examined. Type locality only (7 specimens).
Occurrence and time range. Cheltenhamian; known only from the type locality, occurring with Mesopeplum meringae (Tate) and Mimachlamys asperrima.
Remarks. This species is much the most distinctive of all those we include here in Mesopeplum, and the unusual, crude radial sculpture, the very weak plicae, the very weak internal rib carinae and, in particular, having the opposite valve the more inflated to that of all other Mesopeplum species we are aware of in Australia and New Zealand, suggest that it possibly belongs in a genus distinct from Mesopeplum. However, we have seen relatively little material, the one outcrop it occurs at being now severely overgrown, and almost all material has the exterior coated with pedogenic carbonates, so we know few of the phylogenetically useful characters of M. contrainflatum. At present, we retain it in Mesopeplum, hoping that more, better material will further clarify the identity of the species. It is unusual for a pectinid species to be limited to a single outcrop that seems lithologically indistinguishable from the many others nearby in Gippsland, and the species might well turn out to have a short stratigraphic range.
Etymology. The specific name (Latin, ‘contra’, against or opposite +‘inflatus’, inflated) refers to this species having the opposite relative valve inflation to that of all other species included in the genus.»
ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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