The topological theory and the Volterra process are key tools for the classification of defects in Condensed Mater Physics. We employ the same methods to classify the 2D defects of a 4D maximally symmetric spacetime. These cosmic forms, which are continuous, fall into three classes: i)m-forms, akin to 3D space disclinations, analogous to Kibble's cosmic strings; ii)t-forms, related to hyperbolic rotations; iii)r-forms, never considered so far, related to null rotations. A detailed account of their metrics is presented. There are wedge forms, whose singularities occupy a 2D world sheet, and twist forms, whose singularities occupy a 3D world shell. m-forms are compatible with the cosmological principle of space homogeneity and isotropy, t-and r-forms demand spacetime homogeneity. t-and r-forms are typical of a vacuum obeying the perfect cosmological principle in a de Sitter spacetime. Cosmic forms may assemble into networks generating vanishing curvature.