Correct spelling for the English word "onchus" is [ˈɒnt͡ʃəs], [ˈɒntʃəs], [ˈɒ_n_tʃ_ə_s] (IPA phonetic alphabet).
In geol., a family of sharks whose fossil fin-spines only have been found-they are wide at the base and bent backwards.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.