The Collaborative International Dictionary of English v.0.48:

Rachidian \Ra*chid"i*an\ (r[.a]*k[i^]d"[i^]*an), a. [See
   Rachis.] (Anat. & Zool.)
   Of or pertaining to the rachis; spinal; vertebral. Same as
   Rhachidian.
   [1913 Webster]