microcephalic

from WordNet (r) 3.0 (2006)

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

from 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