Stratigraphy and Mollusca of the Late Permian Senja Formation, Manang area, Nepal Himalaya

Members of the Senja Formation with its bivalves and gastropods are described from the Manang District, north-central Nepal, and compared with faunas from south Tibet and other parts of the Himalaya. The age is deemed to be Late Permian, Changhsingian Stage, on the basis of stratigraphic position above faunas correlated with conodont-bearing faunal assemblages in the Chhidru Formation of the Salt Range, Pakistan, and below faunas containing very late Permian conodonts in Manang and south Tibet. A new genus Yocbelsonellisa, type species Knightites (Retispira) eximia YOCHELSON, 1960 (Bellerophontoidea) is proposed. Species described as new are bivalves Atornodesma astafievae and Aphanaia sakya, and the gastropod species Retispira pucbenpraensis. The nature of zones and subzones, and attendant problems of extent and consistency in age are discussed.