Nostradamus' Lost 58 Quatrains

In 1557, sixteenth century oracle Michele de Nostradame published his opus entitled Les Propheties-- a collection of four-line, rhyming French prophetic verses called "quatrains."  The original collection was supposed to have ten "centuries," composed of 100 quatrains each, for a total of 1,000.  However, Century VII only contains 42 quatrains.  58 quatrains of Nostradamus have never been found.  Where are they and why are they missing?