Source : Webster's Revised Unabridged Dictionary (1913)

Mammalogy \Mam*mal"o*gy\, n. [Mamma breast + -logy: cf. f.
   mammalogie.]
   The science which relates to mammals or the Mammalia. See
   {Mammalia}.

Source : WordNet®

mammalogy
     n : the branch of zoology that studies mammals