Another version of Maha Ganapathy's birth:

Once upon a time, to amuse Herself, Parvati molded the form of an elephant-headed child, using sandal paste mixed with Her perspiration, then She brought it to the Ganga. As soon as the waters submerged the child, He became a resplendent Being. He was Dvaimatura (one of the names ih His Assossthiram), the son born-from-two-mothers, because Parvati and Ganga too, believed that He was their child.

from the Mushika Vahana Stostram:

Umasutham namasyami,
Ganga puthraya they nama,
Omkaraya vashat kara,
Swahakaraya they nama.

Salutations to the Son of Uma,
Salutations to the son of Ganga,
Salutations to one who says,
“Om”, “vashat” and “swaha”.

-KS Balasingam-