. Bulletin of the British Museum (Natural History), Geology. 68 MORRIS, DICKINS AND ASTAFIEVA-URBAITIS Carboniferous, particularly the material in the British Museum (Natural History) described by Hind (1900) as Sanguinolites v- scriptus, show a transition between early species of Myofossa to Cosmomya by the gradual acquisition of typical eccentric zig-zagging ribs. This has led us to place Cosmomya in the Sanguinolitidae. Cosmomya differs from Grammysiopsis Chernychev, 1950 and Pentagrammysia Chernychev, 1950 by the position of these ribs, and it differs from Praeundulomya Dickins, 1957, by t
. Bulletin of the British Museum (Natural History), Geology. 68 MORRIS, DICKINS AND ASTAFIEVA-URBAITIS Carboniferous, particularly the material in the British Museum (Natural History) described by Hind (1900) as Sanguinolites v- scriptus, show a transition between early species of Myofossa to Cosmomya by the gradual acquisition of typical eccentric zig-zagging ribs. This has led us to place Cosmomya in the Sanguinolitidae. Cosmomya differs from Grammysiopsis Chernychev, 1950 and Pentagrammysia Chernychev, 1950 by the position of these ribs, and it differs from Praeundulomya Dickins, 1957, by the shell shape and by lacking the posterior elongate shell thickenings that run close to the hinge in that genus. Pentagrammysia, a genus that has developed the ribbing style of the Mesozoic genus Goniomya indepen- dently, seems to have evolved in central or eastern Asia separately from Cosmomya, but we suggest that it also has common ancestry with a species of Myofossa. Several species of Pentagrammysia are illustrated in Muromzeva's publica- tions (particularly 1974: pis 21-23). In the British species described below, ascending stratigraphical position reflects increasing complexity of ribbing pattern. Early species of the Mesozoic genus Goniomya differ from the type species of Cosmomya, Pentagrammysia and Siphogrammysia in having a sub-umbonal V in the ribbing which slopes downwards and backwards, and is horizontally truncated at least in the umbonal area. We have not been able to decide whether Goniomya is directly descended from one of these Upper Palaeozoic genera with V ribbing, or whether Goniomya has developed this style of ribbing independently. The material figured by Runnegar (1974: pi. 3, figs 1, 2, 11) as Cosmomya maria (Worthen) is better placed in Grammysiopsis, and if both fragments do in fact belong to the same species, they do not differ significantly from G. bisulcatiformis Muromzeva & Kagarmanov (in Muromzeva 1974: 108; pi. 23, figs 21-22). Cosmomya v-s
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