Source : Webster's Revised Unabridged Dictionary (1913)

Sigmodont \Sig"mo*dont\, n. [Gr. ??? sigma (?) + ???, ???, a
   tooth.] (Zo["o]l.)
   Any one of a tribe ({Sigmodontes}) of rodents which includes
   all the indigenous rats and mice of America. So called from
   the form of the ridges of enamel on the crowns of the worn
   molars. Also used adjectively.