Yochelson, E. L., Flower, R. H. & Webers, G. F.: The hearing of the new Late Cambrian monoplacophoran genus Knightoconus upon the origin of the Cephalopoda. Knightoconus, a new genus of the Hypseloconidae (Mollusca: Monoplacophora) from rocks of early Franconian age in Antarctica, is multiseptate. The multiple septa are a criticàl feature to be expected in a form ancestral to cephalopods. Fossil cephalopods, however, invariably have a siphuncle as well as septa; some gastropods, some hyolithids, and some monoplacophorans also have septa but lack a siphuncle. Therefore, only the siphuncle can be considered a unique and particularly significant feature of the cephalopod shell. Hypothetical reconstructions of molluscan anatomy support the notion that cephalopods may have been derived directly from a hypseloconid having a high, slightly curved, multiseptate, bilaterally symmetrical shell, by the subsequent development of a siphuncle.

No abstract

No abstract

The order Ellesmeroceratida is reviewed in terms of three new suborders, their families, and genera. Species are listed under the genera in which they occur, with appropriate indications where transfer from one genus to another has been required. Detailed specific descriptions are confined to new species or to forms discussed and refigured for morphological purposes. Particular attention has been given, wherever material permitted, to the structure of the connecting rings, which are thick and commonly complex and show generally layered structure. Diaphragms, developed in the Plectronoceratina, Ellesmerocera-tidae, and some Proctocycloceratidae, appear to be extensions of the rings. They are suppressed in the Baltoceratidae, and in higher members of that group a ventral rod, presumably originally aragonitic, is developed and is found also in some forms otherwise assignable to the Protocycloceratidae, in which homeomorphic stocks may yet be included. Relationships and evolution of the stock are particularly investigated, and ranges are indicated, with some attention necessarily given to the derived orders Endoceratida, Tarphyceratida, and Michelinoceratida. The table of contents supplies a summary of the taxa to the generic level. A systematic appendix includes inadequately known genera, some forms removed from the Ellesmeroceratida, and some genera of doubtful position. A stratigraphic appendix includes detailed discussions of some sections and problems of ranges of some of the genera and species.

No abstract