<p>This spring, rising on the land of a certain Catus somewhere on the *Quirinal, was the source of the *Petronia Amnis (Festus 39). Until recently it was thought to stem from the Acqua di S. Felice, which rises today in the courtyard of the Palazzo Quirinale (Hülsen, Richardson). Coarelli now locates the <i>fons</i> further to the S, on the <i>collis Mucialis</i> (s.v. Quirinal), where he argues it filled the *Lacus Fundani. The aquifer under this section of the hill, known as the ‘Acque del Grillo’, is very abundant, and a number of its springs were described by Cassio in the 18th c. (<i>Il corso delle acque</i> [1756]). Perhaps the ancient <i>fons</i> was in the vicinity of the spring which now feeds a well at the corner of Via XXIV Maggio and Via Mazarino.</p>