The long limb bones of Desmostylus mirabilis NAGAO from Keton are re-described. The author considered that the description formerly done by SHIKAMA (1966) seems to be in adequate for the mounting skeleton, and he stated somewhat different idea in the evaluation of the morphological characteristics of each bone (Table 1). The common characteristics of the long limb bones of Desmostylus mirabilis are able to be summarized as being stout in general and associated with well-developed epiphyses. Other features are as follows: Humerus: head faced rather backward than upward; major tubercle in lower level than head, protruded foreward feebly; deltoid tuberosity small, projected laterally; body flat fore-and-aft for the width; medial trochlea larger than the lateral in diameter. Forearm: radius and ulna in nearly equal thickness; olecranon thick, bended backward; trochleal axis at right angles to the long axis of carpal articular surface; distal epiphysis well-developed with its articular surface inclined medially. Femur: head spherical, neck constricted clearly along its circumference; major trochanter in lower level than head; minor trochanter in low level, extended long distally; body bended outward, flat fore-and-aft; femoral trochlea hardly concave transversely; intercondylar fossa narrow. Tibia: tibial tuberosity developed remarkably; anterior margin slanted medially to the distal, its free margin leaned laterally in the proximal region; tarsal articular surface inclined anterolaterally.

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Neogene marine sediments and volcanics are widely distributed in the so called "North Fossa Magna" region. The Suwa District, located in the southwest corner of the region, contains the Median Tectonic Line and the Itoigawa-Shizuoka Tectonic Line, which have been active since the Neogene. In terms of geological structure, the Suwa District is divided into two geologically different areas by the Itoigawa-Shizuoka Tectonic Line. 1) The stratigraphy of the Miocene series in the Suwa District is as follows: The Miocene series in the Moriya Area, located on the south side of the Lake Suwa, is called the Moriya subgroup. This subgroup is divided into the lower Moriya formation, which consists of clastic sediments with basal conglomerate, and the upper Gongenzawa formation, which consists of volcanics. The Miocene series in the Yokokawagawa Area, located on the north side of the lake, is known as the Takabochi subgroup. This subgroup is divided into the lower Yokokawagawa formation, which consists of coarse clastic sediments with andesite, and the upper Takabocchi formation, which consists of fine clastic sediments with basalt. The Gongenzawa formation can be correlated with the Yokokawagawa formation, and both can be correlated with the Uchimura formation in the Uchimura region. The Takabocch formation can be correlated with the Kokuzo basalt in the Uchimura region. The intrusions of Tertiary Granitoids, which consist of granodiorite, quartzdiorite, porphyrite, dolerite and rhyolite, are widely developed in both areas. 2) On the basis of the assemblage of secondary minerals identified in the volcanic rocks, the alteration and/or metamorphic area can be divided into the following six zones: I: Mixed layer mineral-Saponite Zone II: Chlorite-Mixed lay V: Actinolite-Chlorite Zone VI: Biotite-Actinolite Zone The zonation and distribution indicate that the alteration and/or metamorphism resulted from thermal or hydrothermal effects on the Tertiary Granitoids. 3) The geologic development of the area, especilly in the Miocene is mentioned below: (1) The Miocene basin was formed by collapses and the creation of sedimentary basin along the Median Tectonic Line in the Moriya stage. (2) Widespread and strong volcanism took place with collapses again in the Uchimura stage. At that time the center of the basin shifted northwards. (3) Plutonism and related metamorphism took place after the subsidence of the basin stopped. (4) After the plutonism an upheaval movement generated the volcano-plutonism at the crest of the area of maximum upheaval. This was accompanied simultaneously by hydrothermal alteration along the faults.

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Rb and Sr concentrations and Sr isotopic compositions (8YSr/86Sr) have been determined for Lower and Middle Miocene basalt, andesitic basalt, andesite and dacite of the Inase, Ishikoshi, Nonodake and Matsushima volcanic rocks in the Kitakami river basin and for Upper Miocene basalt in the Jyoge district to the west of Sendai city. The volcanic rocks of the former region are composed of those belonging to the hypersthenic and pigeonitic rock series proposed by KUNO (1950). The analysed ten volcanic rocks have 87Sr/86Sr ratios ranging from 0.7036 to 0.7040. This range is nearly identical for both series and is similar to that of Lower Miocene Ryozen volcanic rocks from the northeastern part of Fukushima Prefecture. The strontium isotopic data suggest that the primary magmas which produced the volcanic rocks in the Kitakami river basin and the Jyoge district might have been originated from the upper mantle peridotitic material chemically similar to the source material of the Ryozen volcanic rocks. Five new chemical analyses and the published data for these volcanic rocks are not divided into the distribution fields of the hypersthenic and pigeonitic rock series in the AFM, (FeO+Fe2O3×0.9)/MgO-SiO2 and (FeO+Fe2O3×0.9)/MgO-(FeO+Fe2O3×0.9) diagrams. Therefore it is not valid to regard that the hypersthenic and pigeonitic rock series are equivalent respectively to the calc-alkalic and tholeiitic rock series. It is reasonable to consider, in the case of these volcanic rocks, that the essential factor for producing the hypersthenic or pigeonitic rock series is ascribed not to the apperance of magnetite or amphibole with high Fe/Mg ratio at the early stage or the middle stage of fractionation of magma but to a slight difference in the temperature condition of the magma at the stage of crystallization of minerals in the groundmass. There is also no systematic difference in K2O/Na2O ratios between the above two rock series. This evidence and Sr isotopic feature show that the genesis of the volcanic rocks of the hypersthenic rock series can not be ascribed to contamination of granitic material in the magma.

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