Abstract: The specimen described in this paper was collected by Li Gui-yuan from the grey-black middle-thin layered limestone beds of the Middle Triassic Yangliujing Formation at a quarry of Luyin Cement Plant some 150m west of Luyin village in Dingxiao Ecomomie Development Zone，Guizhou. It is a new genus and species of the marine reptile represented by the hind limbs，partial vertebrae and scattered ribs which are briefly described as follows．Order Ichthyosauria(?) Family Dingxisosauridac ram.nov．Genus Dingxiaosaurus gen.nov．Type species Dingxiaosaurus luyinesis sp.nov．Diagnosis Marine reptiles of moderate size Hind limbs longer，larger and paddle-shaped. Femur not expanded distally. Tibia and fibula elongated．Tarsals large，polygonally blocky，and arranged closdy in two rows，three ones in the proximal，and four in the distal row of the tarsus．Digits five．Metatarsals elongated．Phalanges of I —V digits long and number obviously increased．Vertebrae longer and amphicoelous. Etymology Dingxiao and Luyin from the Chinese Phonetic Alphabet of the fossil-producing district and village．Dingxiaosaurus luyinensis sp.nov.(P1．I，fig．1)Holotype Hind limbs，partial vertebrae and scattered ribs of a marine reptile Diagnosis as for the genus(only species)．Locality and horizon Luyin village in Dingxiao Eeomomic Development Zone，Guizhou；Middle Triassic Yangliujing Formation(Ladinian)Description (omitted here)．Remarks The new genus DingMaosaurus，although based on the hind limbs．partial vertebrae and scattered ribs, is highly distinctive．The morphology and the structure of the limb bones show a blend of the characters of the primitive ichthyosaurs and the earliest plesiosaurs．The elongated tibia and fibula are very similar to those of the primitive iehthyosaurs，and the phalanges somewhat similar in shape and number to those of the earliest plesiosaurs．However．the new genus is different form all the known ichthyosaurid and piesiosaurid genera and species，and represents an isolated form of the early marine reptiles．It cannot be placed in any the known family of the ichthyosaurids and the pleslosaurids．Therefore we propose for it a new family Dingxiacsauridae．From the characters of the femur，tibia，fibula，and tarsals，it seems that Dingxiaosaurus shows closer affinity with the primitive ichthysaurs and remoter affinity with the earliest plesiosaurs.