The morphology and mineral composition of jaw apparatuses in Tetragonites glabrus, Gaudryceras denseplcatum, G. tenuiliratum and G. sp. are described. The studied material is well-preserved specimens in some calcareous nodules from the Turonian-Maastrichtian of Hokkaido and Sakhalin. The upper and lower jaws of these species consist of black outer lamella, reduced short inner lamella and covering rostra. The two lamellae are composed of carbonate apatite, and the rostra are calcareous covering. The lamellae and rostra may have been diagenetically replaced from chitin and aragonite respectively, judging from the comparison with modern cephalopod jaws. The calcareous rostra of the upper and lower jaws are analogous to rhyncholite (upper jaw) and conchorhynch (lower jaw) of modern Nautilus instead of coleoids in the internal microstructure, and mineral and chemical composition. These evidences suggest an analogy in feeding habits between tetragonitid ammonites and Nautilus, although the ammonites have larger and stronger jaws.

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This is the fourth of our serial papers on the early Jurassic plants in Japan. In this paper we describe Ctenozamites sarrani belonging to Pteridospermales, and Anomozamites sp. A, Otozamites fujimotoi, O. neiridaniensis sp. nov., Pterophyllum ex gr. propinquum, P. sp. A, Ptilophyllum cfr. cutchense, P. nipponicum sp. nov., P. shinadaniense sp. nov., P. sp. A. and P. sp. B, all belonging to Bennettitales. The occurrence of Ctenozamites and Ptilophyllum is the first record not only in the early Jurassic flora but also in the older Mesozoic floras in Japan.

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Balanus sendaicus was described as a new species by Hatai, Masuda and Noda (1976) based on a single scutum from the Miocene Moniwa Formation, northern Honshu, Japan. However, the scutum deposited in the Saito Ho-on Kai Museum, Sendai, Japan (Hatai et al., 1976) could not be found. Tergum and shell wall, which with scutum comprise the skeletal elements, had not been described. According to my investigation of topotype materials and of materials illustrated by Hatai et al. (1976), the shell walls described and illustrated under the name of B. rostratus by Hatai et al. (1976) belong to B. sendaicus. B. sendaicus, including tergum undescribed by Hatai et al. (1976), is therefore defined in this paper. B. sendaicus is distinct from the already known taxa, as mentioned by Hatai et al. (1976). However, B. sendaicus is closely related to the extinct B. concavus sensu de Alessandri (1906), which flourished in the Mediterranean region (Tethys) from the Oligocene to Pleistocene. B. sendaicus belongs to the B. concavus species group represented by the nominal species based on having transverse septa in longitudinal (parietal) tubes of shell wall and the characteristic ornamentation of scutum. Disappearance of the B. concavus group from the Atlantic and the Mediterranean regions in Pleistocene is mysterious as mentioned by Pilsbry (1916), though "four" species belonging to the B. concavus group are living in the eastern Pacific (Newman, 1979). B. sendaicus is restricted to the Miocene, is the first fossil record of the B. concavus group from the east Asia, and indicates the presence of a connection between the Tethys and the east Asia.

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