The affix ac comes after the word uras when it follows the word prati in a compound and has the sense of the Locative.,
Thus प्रत्युरसम् = उरसि वर्त्तते ॥ This is an Avyayibhava compound, the indeclinable having the force of a vibhakti 2.1.6.
Why do we say \when having the sense of the locative?\ Observe प्रत्युरः = प्रतिगतमुरः ॥,