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Invertebrata |
Department of Geology, Trinity College, Dublin 2, Ireland
Introduction. The old, zoologically based, arrangement of classification of the cephalopods into two major divisions dependent upon the number of gill pairs has been generally superseded by the use of three equal categories representing the nautiloids, ammonoids and coleoids. Subsequent dissatisfaction with this latter widely used scheme has stemmed particularly from the now probably generally accepted notion that the diversity among the nautiloid cephalopods is such as to make their lumping as one taxon equivalent to the relatively uniform Ammonoidea an evident artificiality. To a lesser extent there has been dissatisfaction on the grounds that the loss of the external shell in the coleoids is so significant as to justify their former separation as a group equivalent to the remainder of the cephalopods. Recent reviews of the problem are provided by Donovan (1964), Flower (1961) and Sweet (in Treatise pt K).
The arrangement of two major divisions (Endocochlia and Ectocochlia) for the coleoids and all other cephalopods, respectively, employed in the Russian Osnovy (Ruzhentsev 1962), was not followed in the Treatise on Invertebrate Palaeontology. Sweet (in Treatise pt K), like Flower (1961, 1964a), referred to the difficulty that some Palaeozoic nautiloids display coleoid characteristics.
On the other hand, the Treatise classification follows the pattern in the Osnovy of various subdivisions of the nautiloids regarded as equivalent in status to the ammonoids. That this arrangement is premature is suggested by various lines of reasoning. First, it is by no means clear that the ammonoids should be accorded such high taxonomic status.
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