Definition of "perithecium"
(mycology) An ascocarp shaped like a skittle or ball, distinguished by a small pore, the ostiole, through which the spores are released one by one when ripe.
Quotations
The Erysiphacae may possibly have given rise to the Laboulbeniales, the only other group in which a single daughter cell of the oogonium is responsible for the asci, and perhaps to the lower Pyrenomycetes also; these, like the Erysiphaceae, have regularly arranged asci, and in Chaetomium fimete the perithecium is without an ostiole.
2010, Helen Gwynne-Vaughan, Fungi: Ascomycetes, Ustilaginales, Uredinales, page 54