Collections from the Middle Devonian Onondaga shale and limestone of central Pennsylvania and northeastern West Virginia have provided an abundance of Ostracoda, and the field work has also yielded much information concerning the lithologic features and the relationships of the Onondaga deposits. Descriptions are given of the Onondaga beds observed at 12 localities in Pennsylvania and 2 in West Virginia, and the stratigraphy as seen at these places is discussed.

A total of 40 ostracode species has been recognized in the area. Of these, 3 were previously described, 35 are new. and 2 are represented by material too poor for specific description. The species are distributed through 11 families and 22 genera, 3 of the latter being new. For comparisons with members of this fauna, new figures and descriptions are given of 3 species from the Jeffersonville limestone of Kentucky, and of 1 from the Hamilton shale of Ontario.

Four ostracode zones are recognized in the Onondaga beds near New Bloomfield, Pennsylvania. One of these is widespread; further work is needed to indicate the geographic distribution of the other three. The assemblage as a whole is not close to the known ostracodes of the Onondaga of New York, but further study of the latter will be needed before trustworthy conclusions can be drawn. The ostracodes of the Onondaga of Pennsylvania are clearly distinct from those of the Jeffersonville limestone of Kentucky, suggesting an appreciable difference in age. There are important affinities with ostracodes of the Camden chert of western Tennessee as recently studied by Bassler, but the latter assemblage includes numerous Oriskany elements and is clearly older.

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