There are probably few subjects for the study of which models are more indispensable than the theories of crystal structure ; but the provision of any considerable series of such models is costly, and they require, moreover, a large amount of space for storage. Some interchangeable models, which the author has lately had constructed to illustrate the space-lattices and the regular point-systems of Sohncke, and which may be fitted up and taken to pieces as required, have been found so convenient, both for lectures and for private study, that a short description of them may be found useful, although they differ only in details of construction from those suggested by Sohncke.