This paper is resulting from the study of some Nummulites (N. incrassatus, N. ramondiformis, N. vascus and N. boucheri) of the Ph. de la Harpe collection (Geological Museum, Lausanne). This study appeared necessary, as there seems to be some disagreements among some geologists and paleontologists about the definition of these species. The scope of this work, based chiefly on de la Harpe's publications, was to define the paleontologic characters of N. vascus Joly et Leymerie, and of his megalospheric partner, N. boucheri de la Harpe. These two nummulites have been compared with N. incrassatus de la Harpe. Under this last name, Boussac and some later authors described as well micro- as megalospheric forms. As A and B forms are not always found together, a distinction of them seems to be important. We thus proposed to maintain the name of N. incrassatus de la Harpe, for the microspheric forms. As far as megalospheric forms are concerned, we propose to exhume the name of N. ramondiformis de la Harpe. N. vascus-N. boucheri are very flat nummulites, while N. incrassatus- N. ramondiformis are inflated and characterized by the occurence of a white umbonal masse. Relations and differences between the two groups are illustrated by the two plates and the dimensional chart, all based on de la Harpe's specimens (Geological Museum, Lausanne). The clear distinction of the two groups has a great stratigraphic significance. The group of N. incrassatus-N. ramondiformis is appearing in the middle Eocene (Top Lutetian), and it goes into the Oligocene. N. vascus-N. boucheri, on the other hand, were probably living during oligocene time only. It is our hope, with this new description of the two groups, to help geologists in solving some hard stratigraphic problems, mainly in the alpine nummulitic series.