The Collaborative International Dictionary of English v.0.48:

Trigonid \Tri"go*nid\
   [Webster 1913 Suppl.] Trigonal \Trig"o*nal\, a.
   Having three angles, or corners; triangular; as, a trigonal
   stem, one having tree prominent longitudinal angles.
   [1913 Webster]
   [1913 Webster]