Commentary on the Dream of Scipio

11 Likewise, when uiridia original: "uiridia" (green things) are taken as an ἀντὶ ἐπιθέτου adjective substitute, they make the genitive in -um, such as uiridia prata original: "uiridia prata" (green meadows) and uiridium pratorum original: "uiridium pratorum" (of green meadows). But when we wish to signify the greenness of a place itself, we say uiridiorum original: "uiridiorum", as when it is said: "the beautiful appearance of the green places." For then uiridia is used as a positive noun, not as an incidental quality. Moreover, there was such license among the ancients for this genitive that ASINIVS POLLIO frequently uses uectigaliorum original: "uectigaliorum", even though uectigal original: "uectigal" (tax/revenue) is said no less than uectigalia original: "uectigalia". And although we read,

"And he was carrying the sacred shield (ancile) on his left,"

yet it is also referred to as anciliorum original: "anciliorum". We must therefore see whether variety delighted the ancients more than the idea that the names of festive days were thus called according to an exact rule. For behold, we even find other words, besides the names of solemn days, declined in this way, as the preceding discourse revealed: uiridiorum and uectigaliorum and anciliorum. But I also find the names of the festivals themselves declined according to rule among the ancients, if indeed VARRO says that the day of the Feralium of the Feralia/festival of the dead is named from the feasts carried to the sepulchers. He did not say Feraliorum.