Fossils of the Argonautinae were discovered from the Middle Miocene Bessho Formation of the central Northern Fossa Magna. The occurrence of the Argonauts from the Middle Miocene has never been reported, though the Upper Miocene examples had long been known from several localities in the San'in and Hokuriku areas both of which are on the Japan Sea coast. Some of the fossils obtained this time were found to be Argonauta tokunagai YOKOYAMA. The discovery of Argonauts in the Bessho Formation and the occurrence of Miogypsina sp. from the Moriya Formation (MATSUMARU et al., 1982) reveal the following interpretations. 1) The Moriya Formation which had been regarded as the oldest sediments in the central Fossa Magna and of the earliest Miocene age and also the Bessho Formation which had been considered to be of the Middle Miocene age are the sediments of an age ranging from the late Early Miocene to the early Middle Miocene. It corresponds to certain horizons from N. 8 to N. 9 of BLOW'S zones by planctonic foraminifera. 2) In the central part of the Fossa Magna area, the seas were connected with each other both from the north and the south iust at the stage of the "Nishikurosawa Transgression."

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The authors have been studied the stratigraphy and geological structures of the late Miocene-Pliocene Ashigara group on the northwestern part of Kanagawa Prefecture in central Japan. In the process of the study, the authors found out a small volcanic collapse basin, and named the Yamakita group for the strata burying the basin. The Yamakita group is nearly flat in its structure and abuts to the high angled plane of the unconformity on its base. The marginal basal part of the Yamakita group consists of the huge angular boulder bearing ill-sorted conglomerate is developed, and the upper part of the group is composed of the andesitic pyroclastic rock. From the fact that the Yamakita group is surrounded by the high angle plane of unconformity which links up with the faults with the indurated fault breccia or clay in the Ashigara group, and that the volcanic activity concentrates in the basin of the Yamakita group, the formation process of the basin is concluded as follows: Firstly the expansion of magma resevoir caused the domal uplift of surfacial crust in part where horizontal tensile stress was generated to form many high angled normal faults. Then, the central area of the domal uplift collapsed down to form a basin and the talus deposits filled up a part of the collapse basin. Soon the volcanic activity broke out in this basin and the volcanic materials filled up a part of the basin.

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The properties of the ferromagnetic minerals contained in volcanic ash and pumice samples in tephra layers, Aafa 4, 3, 2, 1 and Mpfa 3, 2b, 1 developed in south eastern part of the Ishikari Low-land have been investigated by means of thermomagnetic and X-ray analyses from a view point of tephrostratigraphy (Fig. 2, 3 and Table 1, 2). The tephras found subsurface in Shizukawa and in three other columnar sections in distant areas in the same Low-land are reasonably well correlated with the typical tephra stratigraphy, mainly on the basis of the thermomagnetic and mineralogical properties of ferromagnetic minerals (Fig. 4, 6, 7, 8). Thus it is confirmed that the thermomagnetic properties of ferromagnetic minerals of those tephras are very helpful for tephra identification, especially when lithology and heavy mineral assemblages of the tephra horizon have been clarified. The results are summarized as below: 1) Characteristics of each tephra are the following; (1) Aafa 4, 3, 2; All Js-T curves are reversible and two Curie temperature ranges from 50°-100℃ (Tc1) and from 525°-540℃ (Tc3) are observed. The substance having Tc1 is attributable to the ilmenite-hematite series minerals and Tc3 is likely to the titanomagnetite (cubic structure minerals). Individual tephra layer is discriminated by the combined features of lithology, heavy mineral assemblages and the form of Js-T curve. (2) Aafa 1, Mpfa 3, 2b, 1; All Js-T curves are not perfectly reversible and show peculiar forms in general. Two Curie temperature ranges from 295°-350℃ (Tc2) and from 530°-545℃ (Tc3) are observed. Tc3s are relatively constant, but the value of Tc2s has unique narrow range for each tephra, namely from 340°-350℃, from 295°-305℃, from 300°-305℃ and 315℃. So thermomagnetic properties provide a diagnostic criterion in identification of these tephras. Mpfa 3 and Mpfa 2b are however very similar as to this property, but these are discriminated by difference in lithology and heavy mineral composition. The ferromagnetic minerals of each tephra are consisted only by cubic stracture minerals. 2) The tephras in the boring core at Shizukawa, Tomakomai City are well identified and correlated on the basis of characteristics above mentioned, together with well known features of Spfa (Fig. 4). Aafa 3, 2, 1 are distinguished in the lower formation of the core, consisted of volcaniclastic sediments. So the formation correlated to the upper Pleistocene, Hongo Formation distributed in an adjacent area (Uma-oi Collaborative Research Group, 1983). 3) It is clear that the tephra correlative to Aafa 2 is intermingled in the ash bearing silt layer, stratigraphically situated under the Shikotsu pyroclastic deposits (Spfa, Spfl) distributed in the central part of the Ishikari Low-land.

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Fossil ophiuroids were discovered from Itsukaichi Basin in the western part of Tokyo, Central Japan. Fossil ophiuroids have been ever found in a few localities in Japan. Writers' fossils might be allied probably to the genus Ophiacantha. About 100 individuals of the genus Ophiacantha were found in the black mudstone rich in marly nodules belonging to the Tateya mudstone member (Itsukaichi Basin Research Group, 1981), whose geological age are the middle Miocene. Relatively well preserved samples are collected in large number, however, most of them show the ventral side and do not show the dorsal side. The ophiuroids are associated with such fossils as Portlandia sp. and some other molluscan fossils, Myctophiformes, fish scales, echinoids and leaves of Fagus sp. and others. Writers described ophiuroids based on the outer moulds of the ventral side as follows. Ophiacantha sp. Disk diameter about 8 mm. The disk is pentagonal, deeply notched at the interbrachial edges. An oral shield, adoral plate, tooth are discernible. Oral slit is very large. The arms constricted at the nodes are about three times as long as the disk diameter. The lateral arm plates are well developed. The first three arm-segments are inserted into the lateral disc. Most of the arm-spines are situated at the lateral arm plates, about two times as long as the arm-segment, erected, and acute at the tip. The arm-spines of well-preserved basal arm-segments are about six in number. Writers' fossils differ from the recent genus Ophiacantha in the fact that the disk of the former is deeply notched at the interbrachial edges. Writers discussed sedimentary environment of the Tateya mudstone member occurred writers' Ophiacantha sp.

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