Note that Achille Richard's dedication makes them both doctors:So he was at the University of Paris to make his doctor.
= "To the memory of Richard Quartin Dillon and Antoine Petit doctors from the Parisian faculty of medicine".Memoriae Richard Quartin Dillon et Antonii Petit doctorum e facultate medicinae parisiensi.
That they were doctors of medicine is also indicated on the title pages of the various volumes of Voyage en Abyssinie exécuté pendant les années 1839, etc., e.g., this one -
A. PETIT et QUARTIN-DILLON, Docteurs-Médecins, Naturalistes du Muséum
If correct, then could this (1836) be him ?