Mesopeplum caroli Iredale, 1929
IREDALE, T. 1929. Mollusca from the continental shelf of eastern Australia. No. 2. Records of the Australian Museum, 17: 157-189. [p. 162, pl. 38, fig. 7-9]
«MESOPEPLUM CAROLI sp. nov.
(Plate xxxviii, figs. 7-9.) This name is proposed for the fine large scallop trawled in 40-80 fathoms off the New South Wales coast, which has been referred to C. hedleyi. I have shown that the latter name was proposed as a new name only for Hedley's C. fenestrata, and that the older name must be resumed. Hedley's species was based on a small shell taken in Port Jackson, and he regarded the fine shell trawled as the adult of his species. I find, however, that his small shell is adult, and differs in shape and solidity from the present form, which is therefore here named as above.
Hedley lumped all the Chlamys-like scallops under the one name, and it is confusing to find quite unlike forms, such as these, associated with the asperrimus series. I had collated a few items when a paper by Marwick¹² on this subject was received, and therefore some of my notes with relation to these southern forms are here included. The Palaearctic forms, both recent and fossil, have been referred to so many little groups that it would be a very unwise policy to attempt to allot our species to these northern groups merely from superficial features. European palreontologists, as well as malacologists, are not agreed, while Dall pointed out the difficulties when American fossil and recent species were studied. Four very distinct series are at once noted, the asperrimus, lividus, bifrons and "hedleyi" groups, and it will clarify matters to designate these groups as genera, so I herewith propose Mimachlamys with P. asperrimus as type, Scaeochlamys with Pecten lividus as type, Equichlamys with P. bifrons as type and Mesopeplum with M. caroli as type. In Mimachlamys the valves are both convex, but the left valve is more convex than the right, the auricles are unequal, the posterior being much smaller than the anterior. The byssal gape is deep and very strong, pectinidial teeth are present, a deeply furrowed fasciole occurring. The sculpture consists of closely scaled numerous radials flanked with subsidiary more delicate riblets, a deep gutter intervening between each group, which becomes filled up with such riblets as maturity is reached. The prodissoconch is smooth, with concentric growth lines, the succeeding sculpture being plain riblets with scratched intervals, the scales developing later. The sculpture on the two valves does not differ appreciably in design. In Scaeochlamys a somewhat different growth sequence can be traced the auricles being more unequal, but otherwise the immature is similar to that of the preceding. The sculpture begins with plain radials, ten to twelve on the left valve but twenty to thirty on the right valve. The interstices between the riblets are threaded radially with irregular scratches, but this interradial sculpture develops into the well-known "Camptonectes" form and then vanishes. On the valves the original ribbing develops in strength, but remains constant in number, scales appearing according to situation, and growing more strongly on the left valve, thus producing a dissimilar effect in the mature stage. The valves also become distorted with age, while the hinge area is more pronounced. Equichlamys, on the other hand, is a much larger flattened inequivalve form with the ears small and subequal, byssal gape small, almost missing, and pectinidial teeth almost obsolete in mature shells, though present in young specimens. Seven to nine distant compound ribs have their broad intervals filled with fine radials, the whole covered with "Camptonectes" sculpture. These large ribs are well marked internally, but, while with age the superficial ribs have a strong tendency to disappearance, the internal ridges become more pronounced, suggesting those of Amusium. The right valve is tightly clasped by the left, with the sides open, another Amusioid feature. Mesopeplum caroli has the 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 the left valve: five prominent distant compound ribs, having the intervals broader than the ribs; the right valve has, similarly, five 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 eight low, flattened semi-equal ribs, in early life scarcely separable but divergent with age. The interstices in the left valve have four to eight fine ridges, those of the right being deeper and with two or three ribs therein. Very fine concentric threads over-run the whole of the left valve, but appear only in the intervals on the right valve, where, on the contrary, concentric growth lines are well marked continuously on the ribs. Height, 45 mm.; length, 48 mm.; thickness of conjoined valves, 21 mm. Type trawled in 55-60 fathoms off Montague Island, New South Wales.» ¹² Marwick.— Trans. New Zeal. Inst., lviii, 4, 1928, pp. 445-456.
TOM IREDALE, 1929
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T. Iredale, 1929, plate 38.
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