Frog-like race that Milk chose over her birth race, and about whose wings she wrote extensively.
The Xenopus are the race Milk chose for herself, giving her the frog half of the half-frog, half-bat creature who greets Carl in the Calligraphy Guild.
Carl recognizes her precisely because of her writing: she wrote extensively about the differences between the wings of her chosen race, the Xenopus, and those of her birth race, the Vesper. It is her wings, and then her name, that tell him who she is.
Frog-like anatomy — damp mottled skin, wide mouth — with wings distinct in structure from those of the Vesper.
Milk's chosen race, and the identifying detail in her published writing.