To achieve an effective in vivo diagnosis of prosencephalic malforma- tions it is necessary to have a systematic method that is closely corre- lated to the gross morphology of the inner and outer surfaces of the brains involved, since those are the features that are demonstrable by pneumography, angiography and computer tomography. An attempt was made to use the existing classification systems, but it then became obvious that they are poorly adapted to the requirements of modem neuroradiology. The many different forms of prosencephaly therefore needed to be re-categorized on the basis of a series of typical speci- mens. The new system is not only simpler and thus more clear-cut from the standpoint of the (in vivo) diagnosis, but in the present author's opinion it also has some advantages as regards the theoretical aspects of the problem. An essential prerequisite for its clarity is that the mal- formations belonging to theyrosepsephalysQe