Dichotochlamys 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. 49]
«Genus Dichotochlamys gen. nov.
Type species. Pecten dichotomalis Tate, 1886, Miocene,
Port Phillip Basin, Victoria. Diagnosis. Chlamydini with RV significantly more inflated than LV; with a moderately wide umbonal angle, length approximately equal to or a little greater than height; retaining a deep byssal notch, long RV anterior auricle and functional ctenolium in adults; with relatively large posterior auricles with a vertical, nearly straight posterior margin and few radial costae; with an almost smooth LV preradial dissoconch, bearing only low commarginal ridges; with weak antimarginal ridgelets developing at the same time as radial costae, strengthening down the disc to form consistent, complex ridgelets on crests of radial costae and intersecting with commarginal ridges to form characteristic frilled, prominent, moderately closely spaced commarginal lamellae; with frequently and closely subdivided and intercalated, numerous, narrow, closely spaced radial costae grouped into low indistinct plicae, initially 5 plicae on LV and 6 on RV, but subdividing at a variable size into more numerous plicae, plicae of all specimens subdividing at least weakly over outer disc of large shells, doubling in number early on disc on many specimens; with costal sculpture of many close, moderately long, narrow, semitubular, ventrally directed spines over outer quarter to half of disc; and with several large, narrow, dorsally directed spines along dorsal margins of RV auricles.
Remarks. Dichotochlamys gen. nov. is similar to Talochlamys (particularly the Miocene T. keiloriana) in many characters, and seems likely to be descended from a Talochlamys stock. The peculiarly fluted commarginal lamellae seen in Dichotochlamys are found also in some species of Talochlamys, and seem likely to be a character expressing a close phylogenetic relationship. Dichotochlamys differs from Talochlamys in its much narrower costae that are much more finely subdivided and intercalated in a regular series of three stages of subdivision down the shell of most large specimens, to produce a much greater number of costae on the outer part of the disc, in having long, prominent dorsal spines, and in being plicate. Also, the two valves are about equally inflated or, if anything, the LV is slightly more inflated than the RV in most species of Talochlamys [although some specimens of T. dichroa (Suter) have the RV more inflated than the LV], whereas in Dicohotochlamys the LV is almost flat and the RV is significantly more inflated than the LV.
Dichotochlamys dichotomalis has a superficial resemblance to species of Mesopeplum (Palliolini), as it shares with that genus the presence of plicae and the LV being less inflated than the RV. However, Dichotochlamys differs from Palliolini in lacking internal rib carinae and in retaining antimarginal ridgelets across the whole disc. The plicae are also less prominent than those of Mesopeplum, and are interpreted as independently evolved in this genus of Chlamydini (as they have also in many other genera of Chlamydini, such as Equichlamys, Notochlamys and Semipallium, among Australian genera). The combination of radial plicae and fine antimarginal ridgelets makes this genus and species immediately recognisable among Australian scallops. At present, we include only the Australian Miocene type species in the genus. It is possible, though, that the two unnamed New Zealand species referred to Notochlamys by Beu & Maxwell (1990; 131, including pl. 9g) belong in Dichotochlamys, as they have relatively weak plicae, and reexamination shows that they lack true shagreen microsculpture, but have the same type of frilled, commarginal lamellae as Dichotochlamys and Talochlamys. They are of Early Oligocene and Middle Miocene age. Etymology. The generic name (dichotomalis + Chlamys) reflects that of the type species, and refers to the series of dichotomous rib subdivisions that take place during ontogeny in most specimens of this species. Gender feminine.»
ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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Dichotochlamys dichotomalis (Tate), type species of Dichotochlamys gen. nov.; A. G. Beu & T. A. Darragh, 2001, Revision of southern Australian Cenozoic fossil Pectinidae, 12A, 12B (lectotype -designated here- of Pecten dichotomalis Tate, RV), 12C-12H.
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