北海道樺戸山地,中新統産のスジイルカ属の新種について

In this paper, a new species of Miocene dolphin Stenella kabatensis n. sp. is described. The material is represented by a fossil skull bone from the Shin-Totsugawa Group at Kokuryo, Uryumura, Uryugun, Hokkaido. The geologic horizon which yielded the material is the Lower Mud Member of Mashike Formation of the Group. The specimen is a well-preserved fossil skull, but excluding the following portions; anterior facial region, supra-occipital bone, posterior part of the frontal bone etc. Accordingly, basing upon some morphological characteristic, the author compared the present specimen with recent and other fossil dolphins. The form of this material suggests the assignment for it to Delphinidae. Succeedingly, it is possible to say that the morphological characters of the tympano-periotic bones of the present material are also common with those of Stenella and Lissodelphis. But it seems that Stenella has much similarity when one disregards the status of larger size of the present. Besides them, it should be noticed that the palatinal portion has nothing of marked longitudinal furrows on it which are usually considered to be an important features of genus Delphinus. In comparing this specimen with recent and other fossil dolphins, some particular characters are able to be enumerated as follows. The swelling of the premaxillae gradually increases to the posterior portion. Asymmetry of the premaxillary is conspicuous, but the degree of it seems to be less distinct when compared with living species. The right side portion of the nasal opening is seemingly broader and less inflated than the left side portion. The ethmoid bone is peculiar in having the remarkable swelling. The maxillae near orifis nasal are broad, and they bend downwardly with steep inclination to the lateral sides, and fairly curve in antero-posterior direction. The maxillae are thick, and the mandible is large and stout. The zygomatic arch is thicker than that of the living species. The palatine bone makes high inclination with the superiorpterygoid. The central part of palatinal portion is flat. There is few information about teeth, but it is possible to say that the tooth of the present specimen, though they were isolated and very small, is slender and curved with conical form. The longitudinal fine striations can not be observed on the tooth crown. As a result of the comparative study on this specimen with other Miocene dolphins reported from California, Maryland and Japan, the author concludes that the present specimen belongs to a new species of genus Stenella.