The Collaborative International Dictionary of English v.0.48:

Stenostome \Sten"o*stome\, a. [Gr. steno`s narrow, little +
   sto`ma mouth.] (Zool.)
   Having a small or narrow mouth; -- said of certain small
   ground snakes (Opoterodonta), which are unable to dilate
   their jaws.
   [1913 Webster]