The Collaborative International Dictionary of English v.0.48:

Pollex \Pol"lex\, n.; pl. Pollices. [the thumb.] (Anat.)
   The first, or preaxial, digit of the fore limb, corresponding
   to the hallux in the hind limb; the thumb. In birds, the
   pollex is the joint which bears the bastard wing.
   [1913 Webster]