829. TINY AMMONITE WORTHOCERAS FROM HOKKAIDO : STUDIES OF CRETACEOUS AMMONITES FORM HOKKAIDO-LX

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  • 829. 北海道産の小型アンモナイト Worthoceras
  • 北海道産の小型アンモナイトWorthoceras〔英文〕
  • ホッカイドウサン ノ コガタ アンモナイト Worthoceras エイブン

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Description

On the basis of several tiny specimens obtained from the mid-Cretaceous (probably Cenomanian) of the Shumarinai area, Hokkaido, by one of us (K. Y.) and Mr. Shunichi Hayashi, we describe a new species under the genus Worthoceras Adkins, 1928. It is similar to W. rochatianus (d'Orbigny), from the Upper Turonian of France, but is distinct in its small size (about a half of that species) with fewer and broader whorls in the spirally coiled early part and the presence of blunt ventrolateral shoulders on the body-chamber which is broader in section. Its suture is well shown, resembling that drawn by Collignon (1929) on an example from the Cenomanian of Madagascar, which may be identical with ours. In an appendix one of us (T. M.) attempts to consider the systematic position of Worthoceras, a highly degenerated ammonite genus of a scaphitoid shape, proposing the family Worthoceratidae nov. It is cryptogenic in our present knowledge but could be presumed as indirectly related to such a genus as Myloceras of the Labeceratidae.

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