Kamasutra 2.9.35

2.9.35

pramṛṣṭakuṇḍalāś cāpi yuvānaḥ paricārakāḥ/
keṣāṃ cid eva kurvanti narāṇām aupariṣṭakam//

The male servants of some men carry on the mouth congress with their masters. It is also practiced by some citizens, who know each other well, among themselves. Some women of the harem, when they are amorous, do the acts of the mouth on the yonis of one another, and some men do the same thing with women. The way of doing this (i.e., of kissing the yoni) should be known from kissing the mouth. When a man and woman lie down in an inverted order, i.e., with the head of the one towards the feet of the other and carry on this congress, it is called the "congress of a crow."