There are seven suburbicarian sees: Ostia, Albano, Porto and Santa Rufina, Palestrina, Sabina and Mentana, Frascati and Velletri. Velletri was united with Ostia from 1150 until 1914, when Pope Pius X separated them again, but decreed that whatever cardinal bishop became Dean of the College of Cardinals would keep the suburbicarian see he already held, adding to it that of Ostia, with the result that there continued to be only six cardinal bishops.
How did Pope Pius X maintain only four cardinal bishops?