Exceptionally well-preserved remains of phosphatized siphuncles were discovered in
  four specimens of a Permian prolecanitid ammonoid Akmilleria electraensis (Plummer
  and Scott) from Buck Mountain, Nevada.  These structures occur as truncated segments
  within the siphuncular tube.  The outer surface of the siphuncle is sculptured by nu-
  merous equally spaced longitudinal ridges and furrows; the ridges represent an infolded
  basement membrane of epithelial cells which corresponds to the distal ends of indivi-
  dual canaliculi between epithelial cells.  In cross-section, the siphuncle of A. electraensis
  consists of a large central vein, possibly two pairs of arteries, porous connective tissue
  with reticulate hemocoelic spaces, and a thin epithelium.  In the presence of two pairs
  of arteries and porous connective tissue, the siphuncle of A. electraensis is more like
  that of Nautilus pompilius than that of Spirula spirula, which has nine arteries and
  dense connective tissue.  However, Nautilus possesses relatively smaller and more nu-
  merous epithelial cells around the siphuncle than does Akmilleria.  These observations
  strongly suggest that the siphuncular epithelium of Akmilleria served as the salt-
  concentrating organ for buoyancy regulation of the living animal, just as in Nautilus
  and Spirula. Key words: Akmilleria, anatomy, Nautilus, Nevada, Permian, Prolecanitid
  ammonoids, siphuncle.