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Memoir 13—Nautiloid Shell Morphology

By R. H. Flower, 1964, 79 pp., 23 figs., 6 plates, 1 glossary, 1 index.

The fundamental parts of the nautiloid shell are (1) conch, (2) septa, (3) connecting ring, (4) siphonal deposits, and (5) cameral deposits. It is important to recognize that the septal neck and connecting ring, ordinarily considered the two hard parts of the siphuncle universally developed, are discrete structures. Classification of siphonal deposits by shape separates homologous structures and unites unrelated ones. A better approach in terms of composition, textures, and mode of secretion is not yet possible, but it is evident that some deposits are true parts of the rings; others are derived from the ring, but now distinct from it; still others are distinct from the ring, cenogenetic in the various orders, and secreted within the siphonal strand; and some are possibly secreted as mantle deposits on its surface. Evidence being ambiguous in some instances, the siphonal deposits are treated in terms of their evolution, in terms of the orders or parts of orders in which they develop. New observations on a number of structures are included.

The present work is a restatement and, to a considerable extent, a revision of the shell morphology of the Nautiloidea. It embodies some new observations and others currently described in several works, now in such an advanced stage that it is difficult to say whether they will precede or follow the present work in publication. The particular need for restatement, however, involves a necessary emphasis of matters of structure rather than of form. The last 15 years of investigation have brought to light new facts indicating increasingly the need of an approach to nautiloid shell morphology in terms of composition, and textures of parts, and, particularly, their mode of secretion. Already, investigation of such matters has contributed to an understanding of relationships where the evidence supplied from form rather than structure seemed ambiguous and open to more than one interpretation, and present findings indicate that further inquiry into such matters will yield even greater rewards.

In a group such as the Nautiloidea, represented by one living genus, and with some highly significant structures known only in long-extinct lineages of the Paleozoic, it may be argued that fossilization forbids our ever being absolutely sure of original composition, that textures may be lost or altered in replacement, and that any possible conclusions as to mode of origin of the various shell parts are necessarily inferential. However, even from the present imperfect observations, it is possible to approach conclusions on these matters as a sort of geometric limit, even though they may never be attained perfectly, and the results have so far been most rewarding. Indeed, one hope leading to publication at the present time, is that others may be influenced to conduct similar investigations, perhaps those having access to material not known or not available to the writer, and fuller understanding of these matters will thus be achieved more quickly.

Certainly it is time that it is realized that the nautiloid shell is an aggregate of various discrete parts. They were not all secreted in the same way and are not all the product of secretion on the surface of a tissue by specialized groups of epithelial cells—regions to which the term mantles has been given, though some are parts of the mantle proper and others are distinct from the original mantle. Further, each of these parts not only shows characteristics of texture and composition, but variations in both can be found as the shell parts are traced in their evolution within the Nautiloidea. For example, it was found that in the Discosorida the connecting ring had various specialized regions differentiated by texture and composition, and the recognition of such rings supplied a criterion of discosorid affinities. Without such evidence, it is possible that the distinctness of the order from the Oncoceratida could not have been demonstrated beyond question, and this evidence permitted the certain assignment of some genera to the Discosordia, the oncoceroid affinities of which could not have been completely disproved otherwise. Certainly it is time that it should be realized that these matters of texture and composition are original, and real and valid criteria in taxonomy; that they may be subject to alteration and even destruction where replacement or recrystallization in fossil material may be advanced is quite aside from the point.

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