No. 5.] 175

48. Discovery of a Desmoceras-Operculum.

By Takumi NAGAO. Department of Geology and Mineralogy, Hokkaido Imperial University, Sapporo. (Comm. by H. YABE, M.I.A., May 12, 1932.) Opercula of ammonites are very rare in the deposits of , and only three cases have been reported by the present writer, in which they were found lying in situ in the body-whorl of ammonite-shells. The first of these is Neoanaptychus tenuiliratus Nagao1 (ad Gaudryceras tenuiliratum Yabe), the second Striaptychus (s. str.) sp.2) (ad an imperfect specimen of Scaphites or Yezoites), and the third Striaptychus (Substriaptychus) yabei Nagao3) (ad Polyptychoceras yabei Nagao and Sasa Ms. nom.), all derived from the Upper Ammonites Beds (Senonian) of Hokkaido. Last summer R. Saito, a student of our Department of Geology and Mineralogy, obtained from the samedeposits a remarkable speci- men of Desmoceras (Kotoceras) semicostatum Yabe Ms. nom.4) with an operculum. In this specimen the operculum lies in the last whorl, with its median line approximately on the median plane of the ammonite-shell, the apex close to the siphonal line and the convex external surface facing posteriorly ; in other words, it is in a disposition just similar to the case of Neoanaptychus tenuiliratus once reported by the writer. The present operculum coincides in form and size with the cross-section of the body-whorl of the shell of Desmoceras, and certainly belongs to the ammonite itself. The operculum consists of a single plate which is rather small (ca. 11mm.high and 9mm. broad), darkcoloured, inverted ovate and truncated ventrally ; it is slightly convex along the median vertical

1)T.Nagao:The Occurrence of Anaptychus-like Bodies in the Upper Cretaceous of Japan. Proc. 7 (1931), 106; Anaptychus and Aptychus Lately Acquird from the Upper Cretaceous of Hokkaido, Japan. Jour. Fac. Sci., Hokkaido, Imp. Univ., Ser. IV, ol.I,No.2,(1931),p.216. 2)T. Nagao :New Discovery of Aptychus in Two Species of Ammonites from the Upper Cretaceous of Japan. Proc. 7 (1931),165; Anaptychus and Aptychus Lately Acquired.Op.cit.,p.219. 3)T.Iiagao:New Discovery of Aptychus. Op.cit.,p.166; Anaptychus and Aptychus Lately Acquired. Op.cit.,p.221. 4)H. Yabe:Cretaceous from Hokkaido (Ms.),(1901). 176 T.NAGAO.[Vol.8, line and strongly transverse to it, the convexity being most marked in the middle part. The ventralmargin probably possessed a faint median notch as suggested by the concentric lines on the surface which make a slight upward curve on the median line. In apical or dorsal view, the whole plate is somewhat crescentic in outline, with its two symmetrical halves or semi-plates expanded laterally like wings. The apex is slightly elevated and imperfectly preserved. Each semi- plate is heart-shaped, higher than broad, with the dorsal margin slightly concave and the lateral one gentlypassing to the ventral in a smooth curve. The surface is ornamented with numerous, crowded, more or less distinct concentric lines parallel to the lateral and ventral margins ; they are slightly elevated, round topped, variably broad, and variably spaced. Locality and geological horizon :The Upper Ammonite Beds (Senonian) exposed along the Kikumenzawa,atributary of the Ikushunbets in the Province of Ishikari, Hokkaido. R.Saito coll. This fossil is attractive in the following three particulars : 1.The ammonite bearing this operculum is "Desmoceras " semicostatum Yabe Ms. nom. which is includedby Yabe in his new genus Kotoceras with the genotype Desmoceras damesi Jimbo from the UpperAmmonite Beds of Hokkaido. We have no previous records of an operculum found within the shell of a Desmoceras in broad sense and also even of that ascribable with any reliability to an am- monite of the genus, aside of Aptychus tumida Coq.1) from the of Algeria. This fossil taken by Trauth2) as the type of his Pteraptychus is characterised by its two valves assuming an Anaptychus-like growth and having a peculiar feather-like ornamentation ; according to Coquand, it was found in association with " Amm9nites " nisus d'Orb., a Desmoceras in broad sense ; but as correctly remarked by Trauth, the evidence is slight for the conclusion of that it really belongs to the ammonite. 2,The present specimen from Japan seems to be an Anaptychus and is much different from the Algerianfossil in form and constitution of the plate. As all the other Palaeozoic and Mesozoic anaptychi, it

1) H. Coquand :Description geologique de la Province de Costantine. Mem. Soc. Geol. , Ser. II, Tom V, (1854), p. 140. 2) F. Trauth : Aptychenstudien. 1.Ueber die Aptychen im Allgemeinen.Ann. naturh. Museums Wien, (1927), pp. 230, 242. No.5.] Discovery of a Desmoceras-Operculum.177 consists of a plate composed in its greater part of a layer of a dark coloured carbonaceous substance altered from an originally chitinous or horny matter. At first sight it appeared to have a faint longitudinal ridge running from the apical region to the middle of the ventral margin and reminded one of an Aptychus ; a closer examination, how- ever, revealed that the ridge is only superficial, being a product certainly due to deformation after its burial in sediment. 3. Anaptychus was not known from the deposits younger than the Liassic until the very recent discovery of Neoanaptychus tenuiliratus from the Upper Cretaceous of Hokkaido to which the specimennow in question is closely similar. The two agree well with each other in essential features, being almost identical in the form, structure and surface ornamentation of their plate and differing only in the shape of the semi-plate, which is cordate in this specimen and oval in the other. Thus, whatever the explanation, the facts are that similar anaptychi occur in a Lytoceran ammonite, Gaudryceras tenuiliratum, and in a Desmoceran form, Kotoceras semicostatum, both of which are found in the same geological formation. Similar opercula may exist among ammonitesof different phylla owing to mere convergence ; otherwise Gaudryceras tenuiliratum and Kotoceras semicostatum are closely related forms. For the time being the writer prefers the first assumption and it is in this sense that his new anaptychus is included into the form-genus Neoanaptychus underthe name Neoanaptychus semicostatus nov. At the end the writer wishes to express his sincere thanks to Prof.H.Yabe of the Institute of Geology and Palaeontology at Sendai for the permission of free use of his private library and valuable help on the manuscript. 178 T.NAGAO.[Vol.8,