The Collaborative International Dictionary of English v.0.48:

Basi- \Ba"si-\
   A combining form, especially in anatomical and botanical
   words, to indicate the base or position at or near a base;
   forming a base; as, basibranchials, the most ventral of the
   cartilages or bones of the branchial arches; basicranial,
   situated at the base of the cranium; basifacial,
   basitemporal, etc.
   [1913 Webster]