Recticytherura ualisicosta gen. et sp. nov. , referred to the Cytheruridae of the Cytheracea, was found in the basal sandy limestone of Member I of the Oligocene Dongying Formation in the Huimin Depression, Shandong Province. This is the first discovery of Cytheruridae fossils from the Eogene in the depression ,even in the Bohai Gulf Basin. In this paper Recticytherura ualisicosta gen. et sp. nov. was described and inferred to be a marine ostracoda based on the characteristics of its hinge, caudal process, radial pore canals and shell wall etc. The assemblage of paragenetic ostracods, the elementary compositions of their shell walls and the carbon oxygen isotopic characteristics of their wall rocks indicate a sedimentary environment of brackish water body. It is inferred according to the spreading of the palaeolake and palaeoclimate that a marining occurred during the early depositional period of Member I of the Dongying Formation in the Huimin Dpression.Description of New Genus and Species Subclass OSTRACODA Latreille, 1806 Order PODOCOPIDA Muller,1894 Suborder PODOCOPINA Sars, 1866 Superfamily CYTHERACEA Baird, 1850 Family CYTHERURIDAE Muller, 1894Recticytherura gen.nov.Etymology: From the Latin recti,straight,and the genus Cytherura, indicating the straight subrectangular caudal process and the resemblance to Cytherura Sars, 1866.Type species; Recticytherura ualisicosta gen. et sp. nov.Diagnosis; Carapace small, subrectangled in lateral view. Dorsal and ventral margins relatively straight and nearly parallel to each other. Anterior end broadly rounded, posterior end with a subrectangular caudal process which is situated centrally to dorsally. Right valve larger. Surface ornamented with ridges and reticulations. Hinge amphidont. Anterior tooth in right valve triangular. Posterior tooth elongate, crenulate. Median element consists of an anteromedian short split socket and a posteromedian fine crenulate groove. Left valve with complementary elements. No vestibula present. Pore canal zone wide. Radial pore canals fine, sinuous, few, branched and generally arranged in groups of several. Adductor muscle scars 4 in vertical row, 2 anterior to them. Eysspots present in some specimens.Comparison: This new genus is similar to Cytherura Sars, 1866 in both the shape and muscle scars, but the latter differs from the former in its left valve larger than the right one and having splitting anterior tooth, smooth median hinge trough and simple radial pore canals.Range and Distribution: Oligocene; Shandong, China.Recticytherura ualisicosta gen. et sp. nov. (Fig. 1; PL I, 1 - 4)Etymology: From Latin, -alts, have or be like, costa, ridge, indicating a U-shaped ridge.Description; Carapace small, subrectangled in lateral view, with dorsum and venter subparallel. The dorsal margin of right valve slightly arched, but the dorsal margin of the left one substraight. Ventral margin faintly concave in front of medial part. Anterior end broadly rounded, slightly sloping at upper part; posterior end with a subrectangled caudal process at the middle to upper part. Right valve larger than left valve, overlapping around the entire margin. The fine ridge of the smaller left valve fits into a contact groove in the larger right valve around the free border. Carapace weakly convex in dorsal view, with maximum width at posterior 1/3. Surface ornamented with ridges and thick reticulations. Two cross ridges on the anterior part and middle-posterior one of a shell, towering over dorsal margin, and linking up with ventral ridge, to forming a U-shaped one. There are some short cross ridges in the U-shaped ridge. The reticulations nearby ridges are generally parallel to the ridges.Locality and Horizon: Linyi region in the Huimin Depression, Shandong Province; Member H of the Oligocene Dongying Formation.