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Published online by Cambridge University Press: 01 April 1999
We characterise all maximal convex [lscr ]-subgroups of A(Ω), the automorphism group of a doubly transitive chain Ω of countable coterminality. We then determine, up to isomorphism, all doubly transitive actions of the [lscr ]-group A(Ω). Finally, assuming the Continuum Hypothesis, we show that there exists an unbounded chain of proper prime subgroups of A(Ω).