The boundaries of the IX regio of Rome are re-examined, revealing possible chronological changes. The figure of XXXII D feet assigned to the perimeter in the Regionary Catalogues must perhaps be changed to XXII D. A reconstruction of the geography of the southern part of the Campus Martius (palus Caprae, in a wide area between the Tiber and the Pantheon; amnis Petronia, whose course was perhaps further south than has been thought previously) can be helpful in considering several topographical problems of the region, for instance the arbor caprificus and the aedes Volcani, which probably stood at the edge of the marsh and beyond the amnis Petronia. Martial's mention of Europa, generally related to a porticus Europae, could, on the contrary, concern the famous picture of Antiphilos in the porticus Pompei.