. Bulletin of the British Museum (Natural History), Geology. Fig. 25 Dyasmya elegans (King). Upper Permian, Magnesian Lime- stone, Claxheugh Quarry, Durham; C. T. Trechmann collection. Fig. 25a, BM PL5004, top view; Fig. 25b, BM PL5005, left side; both Fig. 26 Wilkingia sp. Lower Carboniferous, Kansas. USNM, no register number; Fig. 26a, left side; Fig. 26b, ventral view; both approx. xO-8. genera Mya and Panopea, differing somewhat in these features from the type species of Wilkingia. We have been unable to find additional material with these important details preserved. It may be import
. Bulletin of the British Museum (Natural History), Geology. Fig. 25 Dyasmya elegans (King). Upper Permian, Magnesian Lime- stone, Claxheugh Quarry, Durham; C. T. Trechmann collection. Fig. 25a, BM PL5004, top view; Fig. 25b, BM PL5005, left side; both Fig. 26 Wilkingia sp. Lower Carboniferous, Kansas. USNM, no register number; Fig. 26a, left side; Fig. 26b, ventral view; both approx. xO-8. genera Mya and Panopea, differing somewhat in these features from the type species of Wilkingia. We have been unable to find additional material with these important details preserved. It may be important that the form of the escutcheon resembles that of W. regularis without the lateral separation MORRIS, DICKINS AND ASTAFIEVA-URBAIT1S of the dorsal shell margins typical of Pleuromya and other Myacea. Genus PRAEUNDULOMYA Dickins, 1957 Type species. Praeundulomya concentrica Dickins 1957, by original designation. Description. Transversely elongate, with well-developed escutcheon behind umbones in the form of a flat marginal area. The ligament proper was apparently lodged in a relatively short groove on the proximal edge of narrow nymphs close behind the umbones. The flat marginal area was probably covered and joined by periostracum, continuous with the ligament (see p. 54). The muscle scars are very shallowly impressed and a deep pallial sinus is visible in P. maxima and P. subcuneata (but not the type species). There are one or two ribs running below the hinge posterior to the umbones. These appear as grooves on the steinkern or composite mould, and are better developed than in Wilkingia. In the Permian species P. concentrica, the two posterior grooves are as well developed as they are in Undulomya. In outline from above, bivalved specimens have a distinctive tapering shape. Praeundulomya is distinguished from Wilkingia essentially by the shape; in Praeundulomya the ventral and dorsal margins are sub-parallel with only a very slight subumbonal sulcus, whereas Wilkingia has a
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