The Collaborative International Dictionary of English v.0.48:

Microcephalic \Mi`cro*ce*phal"ic\, Microcephalous
\Mi`cro*ceph"a*lous\, a. [Micro- + cephalic, cephalous.] (Anat.)
   Having a small head; having the cranial cavity small; --
   opposed to megacephalic.
   [1913 Webster] microchemical

WordNet (r) 3.0 (2006):

microcephalic
    adj 1: having an abnormally small head and underdeveloped brain;
           "a nanocephalic dwarf" [syn: microcephalic,
           microcephalous, nanocephalic]