Abstract
Aptychi seem to be extremely rare in the Japanese Jurassic, Cornaptychus nagatoensis NAGAO from the Lias of the province of Nagato being the unique example hitherto recorded. Recently Prof. H. YABE of the Institute of Geology and Palaeontology, Tohoku Imperial University, kindly submitted for determination to the. writer an aptychus obtained from the Upper Jurassic rocks in the southern part of the Kitakami Mountainland ill Northern Honsyii. The specimen is, as described below, represented by a cast of a two-valved operculum with the ornamentation of either outer or inner surface impressed on the matrix. The parent ammonite is unknown at present. In the district where this operculum has been obtained, the Upper Jurassic rocks composed mainly of dark coloured hard slaty shale alternated with sandstone are developed. On the label sent to the writer together with the specimen is recorded that it had been found at a locality with, separately, a specimen of Perisphinctes (Aulacosphinctes?) sp. No ammonite of other genera is yet reported to occur in this complex, it will be very natural to suppose that this operculum belongs to some form of this genus.
