The Collaborative International Dictionary of English v.0.48:

Membranous \Mem"bra*nous\, a. [Cf. F. membraneux.]
   1. Pertaining to, consisting of, or resembling, membrane; as,
      a membranous covering or lining.
      [1913 Webster]

   2. (Bot.) Membranaceous.
      [1913 Webster]

   Membranous croup (Med.), true croup. See Croup.
      [1913 Webster]

WordNet (r) 3.0 (2006):

membranous
    adj 1: relating to or made of or similar to a membrane;
           "membranous lining"
    2: characterized by formation of a membrane (or something
       resembling a membrane); "membranous gastritis" [syn:
       membranous, membrane-forming]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "membranous":
   filmy, foliaceous, foliated, in layers, lamellar, lamellate,
   lamellated, lamelliform, laminate, laminated, laminous, leaflike,
   leafy, scummy, spathic, spathose, stratified, stratiform,
   three-ply, two-ply