Alagoa, E. J. (1968). "The Use of Oral Literary Data for History: Examples from Niger Delta Proverbs". The Journal of American Folklore, 81(321), 235–242. https://doi.org/10.2307/537543.
oup.com
oxfordindex.oup.com
"The second expedition". oxfordindex.oup.com. Oxford University Press. Retrieved 27 October 2017.