This tomb, placed in an eminent position at the intersection between Via Laurentina and a diverticulum of that road, is to date the oldest one documented in the necropolis (50-30 BC): it is an enclosure in opus reticulatum that encloses a square monument lined with travertine on the front and tuff stone on the sides, whose coping (perhaps tholos or altar style) was lost. Immediately opposite is enclosure 21, on whose façade is the inscription on a travertine slab that, in addition to indicating the name of the deceased, D(ecimus) Nonius Philomusus, bears the tomb’s dimensions (15 feet for the façade, 20 feet in depth).