抄録
The complete skeleton of Desmostylus is known only from Japan. It was found in Keton, Shisuka-machi, Shisuka-gun, South Sakhalin in 1933. The skull and post-cranial skeleton were described before (IJIRI and KAMEI, 1961 ; SHIKAMA, 1966) but the vertebrae, ribs, scapula and hip bone remain unexamined to date. In the present paper the atlas and nine thoracic vertebrae of the Keton specimen are described and the vertebral formula of the studied part is discussed. The characteristics of the thoracic vertebrae are as follows: Body compressed fore-andaft, low and transversely wide; size of vertebral foramen not so larger than the body, so that radices of vertebral arch projected due upward; cranial margin of vertebral arch slightly reduced; cranial articular processes not distinct; mammillary processes protruded forward dorso-laterally; caudal ones projected latero-posteriorly; cranial vertebral notches obscure, caudal ones deep and U-shaped; pedicles of vertebral arch not perforated; grooves for spinal nerves stretched slightly down-, out and backward; transverse processes short, projected more or less upward and due laterally; spinous process short, not inclined in the posterior thoracics; cross section triangular, cranial margin sharp, caudal one broad. Previous studies misregarded the left fifth metacarpus as the left first rib, and insisted that the thoracic vertebrae numbered 14 or 15 (NAGAO, 1941) or 14 (SHIKAMA, 1966), but it should be concluded that they number 13. The exact determination of the position and order nomination of thoracic vertebrae depends on the correctly recognized number of the lumbar vertebrae. Although earlier studies concluded that the lumbar vertebrae numbered 4 (NAGAO, 1941), or 6 (SHIKAMA, 1966), the Keton specimen proves impossible to distinguish the posterior thoracic vertebrae from the lumbar vertebrae by such characters as costal fovea or transverse process due to poor preservation. Fortunately, since a thoracic vertebra was found with its costal neck preserving the original position, the order nomination of thoracic vertebrae was able to be determined. Thus it can be concluded that Desmostylus has 13 thoracic and 4 lumbar vertebrae.
