. Bulletin. Natural history; Natuurlijke historie. ORDOVICIAN APPALACHIAN ECOLOGY 33 II are not randomly mixed; rather, they occur in a characteristic stratigraphic order with Tancrediopsis and Ischyrodonta overlying Ambonychia and Modiolopsis (Fig. 18). In addition Modiolopsis modiolaris dominates bivalve province II in the more easterly exposures in central Virginia; Tancrediopsis cuneata and Ambonychia prae- cursa are dominant in south-central Pennsylvania and eastern West Virginia. Tancre- diopsis cuneata and Ischyrodonta truncata are only occasionally found on the same BIVALVE ZOOGEOGRAPH
. Bulletin. Natural history; Natuurlijke historie. ORDOVICIAN APPALACHIAN ECOLOGY 33 II are not randomly mixed; rather, they occur in a characteristic stratigraphic order with Tancrediopsis and Ischyrodonta overlying Ambonychia and Modiolopsis (Fig. 18). In addition Modiolopsis modiolaris dominates bivalve province II in the more easterly exposures in central Virginia; Tancrediopsis cuneata and Ambonychia prae- cursa are dominant in south-central Pennsylvania and eastern West Virginia. Tancre- diopsis cuneata and Ischyrodonta truncata are only occasionally found on the same BIVALVE ZOOGEOGRAPHY. FIG. 17. A generalized outline of the distribution of the most common bivalve mollusc species in the central Appalachian Upper Ordovician. The three major bivalve faunal provinces are: L Lyrodesma poststriatum Ambonychia radiata Ctenodonta? pulchella Praenucula levata Tancrediopsis cuneata Ambonychia praecursa Modiolopsis modiolaris Ischyrodonta truncata Pterinea {Caritodens) demissa Note as was the case with the brachiopod faunal provinces overlay may not indicate faunal mixing since the faunas are separated stratigraphically, see Fig. 18. n. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Peabody Museum of Natural History. New Haven : The Museum
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