De Verborum Significatu
Sextus Pompeius Festus
Rutundem aedem Vestae Numa Pompilius rex Romanorum consecrasse videtur, quod eandem esse terram, qua vita hominum sustentaretur, crediderit: eamque pilae formam esse, ut sui simili templo dea coleretur.
Numa Pompilius, the king of the Romans, seems to have consecrated a circular temple to Vesta, because he believed that she was the earth, by which human life is sustained: and that it was the shape of a mortar, so that the goddess might be worshipped in a temple similar to herself.
Translation by Jane W. Crawford, © 2001.