“Some construe the individual species by dividing the group into two differentiae.  In one way this is not easy, in another impossible.  For of some there will be only one differentia, but the other terms will be superfluous, such as in the case of footed, two-footed, footed with parted toes.  For this last differentia alone is proper.”
(Aristotle, Parts of Animals, 642b5-9 Greek follows below)

This begins that section of the Parts of Animals concerning a critique of the Platonic method of division, here namely in light of animal divisions.  This initial critique begins with bifurcation, that is, division of animals into X or not-X.  (This is meant to include not only antinomies, i.e. winged and wingless, but also opposites such as black and white, as well as other empirically inducted differences.)  Since this small passage kicks off the discussion, it would desirable to obtain some clarity about it. Exampling this method, by taking the X route, Aristotle proceeds from “footed,” leaving “non-footed” to the side.  He then further divides “footed” into “two-footed” and “footed with parted toes.”  “Footed with parted toes” is said to be proper or valid (κυρία).  This presumably means that the terms “footed” and “two-footed” are unnecessary, as they are included in the concept of “footed with parted toes.”  This is what Aristotle means by “some terms being superfluous (περίεργα).”

However a number of problems remain:
It is not true that the concept of “footed with parted toes” includes “two footed” and “footed.” viz. lizards

It is also clear from his use of the term elsewhere, that “footed with parted toes (σχιζόπους)” is not meant to designate any particular species alone, such as humans (cf. HA 593a28, concerning birds).

Perhaps this introductory polemic is intended by Aristotle to conjure up the Academic chestnut that man is a featherless biped, showing here, of course, both how absurd that notion is and its genesis in a faulty method of division. (See Statemans 266e for the origin of the featherless biped.)

Λαμβάνουσι δ’ ἔνιοι τὸ καθ’ ἕκαστον, διαιρούμενοι τὸ (5)
γένος εἰς δύο διαφοράς. Τοῦτο δ’ ἐστὶ τῇ μὲν οὐ ῥᾴδιον, τῇ δὲ
ἀδύνατον. Ἐνίων γὰρ ἔσται διαφορὰ μία μόνη, τὰ δ’ ἄλλα
περίεργα, οἷον ὑπόπουν, δίπουν, σχιζόπουν· αὕτη γὰρ
μόνη κυρία. Εἰ δὲ μή, ταὐτὸν πολλάκις ἀναγκαῖον λέγειν.