. Bulletin of the British Museum (Natural History), Geology. 12 SHELL STRUCTURE the Sowerbyellidae did the lophophore evolve into a quadrilobed structure. In various Ordovician species of Anoptamhonites and Bimuria there is muscle scar evidence for the dorsal adductors being restricted posteriorly, in a more traditional position, and these scars are divided by raised areas which may be homologized with Cocks' 'outer side septa' and possibly with the anderidia of chonetaceans. It is believed that the Sowerbyellidae, particularly Eopledodonta and Pledodonta, were specialized plectambonitaceans w
. Bulletin of the British Museum (Natural History), Geology. 12 SHELL STRUCTURE the Sowerbyellidae did the lophophore evolve into a quadrilobed structure. In various Ordovician species of Anoptamhonites and Bimuria there is muscle scar evidence for the dorsal adductors being restricted posteriorly, in a more traditional position, and these scars are divided by raised areas which may be homologized with Cocks' 'outer side septa' and possibly with the anderidia of chonetaceans. It is believed that the Sowerbyellidae, particularly Eopledodonta and Pledodonta, were specialized plectambonitaceans which nevertheless retained essentially normal systems of feeding. Their 'cousins', the Aegiromeninae remained more generalized and are morphologically more suitable to have provided the stock from which chonetaceans evolved. The Aegiromeninae tend to be small-sized shells, commonly about lo mm. wide, gently concavo-convex with shorter interareas and more regular ribbing than the Sowerbyellinae. Socket ridges are reduced and the dorsal median septum does not extend posteriorly to the cardinal process but appears to be flanked by the adductor. Fig. 4. Stylized illustration, based on Bimuria siphonata Cooper, from the mid-Ordovician of Pratt Ferry, Alabama, showing the internal dorsal valve morphology on the left with the inferred lophophore and main muscles on the right (red). The blue arrows indicate the main circulation of water through the lophophore, and this is further illustrated by the small diagram of an open shell, viewed posteriorly, on the right. (The dorsal valve is uppermost and water enters from the front) — anterior adductor scar; - ventral attachment area of adductor muscle; - brachial platform; - brachial ridge; did. - diductor muscle; m. - mouth; - posterior adductor scar; - socket Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and
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