† penˈtagonous,a.Obs. [f. L. pentagōn-us(see pentagon) + -ous.] = pentagonal. 1661Lovell Hist.Anim. & Min.Introd., Amongst Fishes, The Cartilagineous, are plaine or long:..The heart is pentagonous.1673Phil.Trans. VIII. 6188. 1761 Ellis ibid. LII. 358 It is formed of pentagonous joints, or vertebræ.