parathecium (noun, pl. parathecia;ascocarp term; fromGreek "para" for beside, along the side) – the lateral part of the proper exciple. Several mycological glossaries generally use the definition "outer hyphal layer" for the parathecium. Unfortunately, this definition is only applicable, if no additional thalline part is present along the outside of an ascocarp. In lichenized ascomycetes with a lecanorine apothecium the parathecium is not the outermost hyphal layer. Instead, the parathecium is enclosed by an additional layer, which is ontogenetically derived from the thallus. In this case the parathecium it is enclosed by a thalline exciple or amphithecium. In perithecioid ascocarps the distinction between the lateral part of the proper exciple, the parathecium, and the lower part, the hypothecium, is obsolete. Perithecioid ascocarps of lichenized fungi are also frequently characterized by an additional layer of thalline hyphae, typically forming a shield-like structure called involucrellum. This involucrellum is not part of the proper exciple (or parathecium) and instead can be interpreted as the thalline exciple (or amphithecium). This is:part of ascocarp.