Analysis of the stratigraphic occurrences of 23 species of podocope, metacope, and platycope ostracods in Wenlockian strata of the type Wenlock area of the Welsh Borderland demonstrates the following. (1) Five species are restricted to the lower Wenlockian; four of these range from the centrifugus Biozone through the rigidus Biozone, and one ranges from the centrifugus Biozone through the ellesae Biozone. (2) Five species range throughout or nearly throughout the Wenlockian sequence. (3) Thirteen species are restricted to the upper Wenlockian; eleven of these range from the lundgreni Biozone through the lundensis Biozone, one ranges from the nassa Biozone through the lundensis Biozone, and one ranges from the lundgreni Biozone through the nassa Biozone.

These observations, which are based on the identification of more than 11,000 ostracods, indicate a significant change in the ostracod fauna in the middle of the Coalbrookdale Formation, i.e., in the middle of the type Wenlock Series or near the Sheinwoodian-Homerian boundary. A similar change occurs in the brachiopod succession in the same interval. Because the change in the ostracod fauna is relatively abrupt (i.e., within two graptolite biozones), we believe that it was induced by an environmental change which did not significantly affect the lithology of the stratigraphic interval involved. We conclude that the interval near the Sheinwoodian-Homerian boundary in the type Wenlock area represents the time of maximum Wenlockian transgression, after which regression and shallowing occurred.

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