Abstract
Examination of specimens from the herbarium of C. A. Agardh at Lund in Sweden has resulted in the choice of a lectotype for Melosira nummuloides C.Ag., the type species of the genus. A specimen has been placed in the diatom collection at the British Museum (Natural History). The structure of the frustule of fresh and cultured material has been examined in detail and a revised description of the species is presented in anticipation of a wider study of the genus. The type of valve structure is discussed and the function of some of the siliceous structures is also considered.