As late as 1891 Rogers in his technical glossary recorded only the form incunabulum: Rogers, Walter Thomas (1891). A Manual of Bibliography (2nd ed.). London: H. Grevel. էջ 195.
J. Green, F. McIntyre, P. Needham (2011), "The Shape of Incunable Survival and Statistical Estimation of Lost Editions", Papers of the Bibliographical Society of America 105 (2), pp. 141-175. doi:https://doi.org/10.1086/680773