Aladžova-Hriščeva, K., and J. Pavšič. “Nummulites and Calcareous Nannofossils from Lower Eocene Beds in the Area of Veliko Târnovo (North Bulgaria)”. Geologija, vol. 31, no. 1, June 1988, pp. 209-17, https://www.geologija-revija.si/index.php/geologija/article/view/609.