[St.Chuck]

The Pius XII quotation from Mediator Dei refers only to minor adjustments to the ancient liturgy.

Are we reading the same thing?

" the Sovereign Pontiff alone enjoys the right to recognize and establish any practice touching the worship of God, to introduce and approve new rites, as also to modify those he judges to require modification."

I guess the reference to minor adjustments is in the footnotes available to a select few.

[ultima ratio]

If you think Pius XII meant by this a pope could write his own Mass, you would have to explain further why he would have issued an entire encyclical just to justify his few minor changes in the existing rite. Clearly he was not talking about inventing Masses. By the "introduction" of "new rites" he meant the new missals containing these adjustments. He did not mean INVENTING NOVELTIES. How do we know? Because he goes through great trouble explaining how the Mass had evolved organically over the millenia under the guidance of the Holy Spirit. This notion of organic evolution totally contradicts any notion of introducing anything new without its having proceeded from what came before--in the fashion of the Novus Ordo which was, as Cardinal Ratzinger puts it, "fabricated".