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It has long been recognized that cephalopods are among the best of index fossils, and the Late Paleozoic and the Mesozoic strata are almost universally zoned on the basis of ammonoids. In Europe the Devonian has similarly been subdivided into ammonoid zones, but in America comparatively little use has been made of these early ammonoids for stratigraphic purposes. This latter circumstance appears to be the result of two factors: first, ammonoids are not abundant in the Devonian rocks of America, and second, the literature relating to Devonian ammonoids is not readily available to most American paleontologists, for the bulk of it has been published in Europe and even that which deals particularly with American forms is widely scattered and is, in a large part, more or less out of date. The present publication is intended to overcome the latter of these handicaps to the use of ammonoids for the correlation of American Devonian strata; also, it is hoped that this study will stimulate a search for more and better ammonoids in the Devonian rocks of America and that the other handicap to their use for stratigraphic purposes will thereby be lessened.

DISTRIBUTION OF DEVONIAN AMMONOIDS

During the Devonian, as in subsequent times, ammonoids were widely distributed geographically, and closely similar, if not identical, species are known to occur in both the Eastern and the Western hemispheres. The Devonian rocks of Europe have yielded ammonoids in greater abundance and in greater variety than have those of any other continent, but locally, . . .

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