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The Phragmocone of Ecdyceras

By A. K. Armstrong, 1962, 99 pp., 41 figs., 12 plates, 1 index.

Ecdyceras proves to have a rather long phragmocone, at the base of which a rather flat, thick septum suggests loss of earlier growth stages by truncation. This shallow basal septum is followed abruptly by very deeply curved septa, their edges extended far forward. Episeptal deposits, thin centrally and thick peripherally, result in a peripheral zone in which the shell material was essentially solid and an axial zone in which cup-shaped cavities remain in the camerae; with growth, the axial zone widens adorally at the expense of the peripheral zone. The siphuncle, beyond a basal arcuate stage, is tubular and thick-walled, with diaphragms, later becoming thin-walled and showing slightly fusiform segments. Septa are thin, the necks ambiguous from the present material, but evidently vestigial.

Chazyan material studied is indecisive at the specific level, but indicates that the extended septal margins of young stages are responsible for the apparent thickening of the base of the living chamber on the basis of which Pachecdyceras was distinguished. A new species, E. expansum is described from the Viola Limestone of Oklahoma. Ecdyceras is remote in structure from other cephalopods, requiring its separation into a family and an order by itself. Origin is obscure, but only derivation from the Ellesmerocertaida seems possible.

This paper is devoted primarily to the description of some remarkable phragmocones which are plainly to be attributed to the genus Ecdyceras, and which result in a drastic revision of previous concepts of the morphology and taxonomic position of that genus. These phragmocones show a structure so completely alien to that of other known orthoconic nautiloids-indeed, to all other nautiloids-that the genus is isolated in evolution, and forms connecting it with simpler nautiloids are as yet unknown. The material here described is from the type section of the Chazyan, southwest of Chazy, New York. Some problems attend the detailed interpretation of the section, but it will suffice to note that the phragmocones were collected in sight of the locality which yielded the original materials of Ecdyceras, and the occurrences are separated stratigraphically by somewhat less than 60 ft. The matter involves some questions of interpretation, discussed more fully in the text.

The phragmocones show septa which are extremely prolonged forward at their margins, and indeed, there is developed a peripheral zone in which episeptal deposits fill completely the narrow spaces between the steeply inclined septa, except in adoral camerae where the deposits are immature or wanting altogether. The result is an appearance in sections so foreign to that of other cephalopods that without strong evidence to the contrary, one could suggest that these forms belonged to some other fossil group. A casual glance at such a specimen as is shown in the lower part of an included plate and figure will show a general resemblance of this part of the section to that of a Beatricea, though the bottom of the figure as oriented here would be interpreted as the top of the stromatoporid. The anterior part of the same figure resembles a section through a belemnite shell, showing the phragmocone surrounded by a rostrum.

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