Kehr states that Velletri was never one of the seven bishoprics which carried the title of cardinal. Paul Fridolin Kehr (1907). Italia pontificia: Lativm (in Latin). Vol. II: Latium. Berlin: Weidmann. pp. 101–102. Giovanni must have been a bishop of Velletri who was created a cardinal in a separate action.
Bonizo of Sutri, "ad amicum" VI (Monumenta Germaniae historica: Libelli de lite imperatorum et pontificum saeculis XI. et XII. conscripti (in German and Latin). Vol. I. Hannover: Hahn. 1891. p. 592.): "Scio, fratres, quia post mortem meam exsurgent viri ex vobis amantes semetipsos, qui non per decreta sanctorum patrum, sed per laicas personas hanc sedem arripient." Quo audito omnes pariter negavere et sacramento dato in manu pape, sese mutuo ligavere nunquam se aliter, quam decreta sanctorum patrum exigunt, ad pontificalem ascendere sedem vel ascendentibus consentire.
Paul Fridolin Kehr (1907). Italia pontificia: Lativm (in Latin). Vol. II: Latium. Berlin: Weidmann. pp. 101–102.: "...inde a saec. XI ex. ab Ostiensibus episcopis administrata, circiter a. 1150 ab Eugenio III cum Ostiensi episcopatu unitur, ita ut deinde episcopus s. Ostiensis et Veliternensis ecclesiae appelletur."
Louis Duchesne (1892). Le Liber pontificalis (in Latin and French). Vol. Tome II. Paris: E. Thorin. p. 279. Cf. Watterich, p. 203.
