The material upon which this paper is based was collected by Messrs. S. C. Kwan and S. T.Che from the Zhuozishan limestone in southwestern part of Inner Mongolia during their fieldinvestigation in 1953. A part of specimens were procured by Messrs. Y. H. Lu, A. T. Mu andJ. T. Chang from the same district in 1954. The lower Ordovician strata of the Zhuozishan region was divided by Messrs. S. C. Kwanand S. T. Che (1955, Acta Geological Sinica, vol. 35, no. 2, p. 98--101) into three parts,namely: the Sandaokan formation in the lower part, the Zhuozishan limestone in the middle partand the Klimli limestones in the upper part. The gastropods are largely procured from theZhuozishan limestone. This limestone series consists mainly of greyish blue, massive limestons