白亜紀アンモナイト Puzosia kuratai Tokunaga and Shimizu, 1926 について

抄録

Puzosia kuratai Tokunaga and Shimizu, 1926 was established on two large specimens (i.e. syntypes), from the fossiliferous conglomerate in the lower part (Coniacian) of the Futaba Group at Oriki (Fig. 1), of which the figured one (Figs. 2, 5) is designated in this paper as the lectotype. The two specimens were bombed away during World War II, but a plastercast (Fig. 3) of the lectotype remains in Tohoku University. It was described originally to be compressed, flat sided and unornamented. Should it be truly so, then it would represent a distinct species, to which a form from the Coniacian of Hokkaido provisionally called P. cf. manasoaensis Collignon by Matsumoto and Kera could be referred. Recently, one of us (T. W.) has obtained from the type locality a fairly large specimen (Fig. 6) which looks similar to the lectotype of P. kuratai. It is somewhat distorted and partly eroded. Judging from the characters observed on its better preserved part (Figs. 7, 8), it is regarded as a macroconch of Mesopuzosia yubarensis (Jimbo), although its body chamber is unpreserved. Moreover, a smaller specimen (Fig. 9) was obtained from the same rock, which is undoubtedly identified with M. yubarensis (microconch). As the lectotype is a somewhat secondarily compressed and partly eroded internal mould, on which the external shell is almost unpreserved, its flat flanks and smooth looking surface may be due to this unfavourable preservation. Its surviving fraction of the outer whorl has a blunt bulge on a part of the flank (Fig. 4). This feature is similar to the appearance of periodic bulges on the flanks in adult macroconchs of M. yubarensis. Thus P. kuratai could possibly be a synonym of M. yubarensis. It should remain, however, as nomen dubium, until a well preserved topotype which fits the original description could be obtained.