curious fossil

From
Date
 Ok, I give up.  Some of my students were playing Stump the Professor this
  week, and one of them brought in a fossil I can't identify.  I'm sure it's
  something very commonplace and that I'm overlooking some important feature.
  But after scanning several ID manuals for invertebrate fossils, I can't
  find anything that fits.  I am sure, however, that it is not vertebrate in
  origin.

  I'll try to give a verbal description (it's fairly complicated), and would
  appreciate any hints from other contributors to this list.

  The fossil is from an unknown locatlity and of unknown age.  It appears to
  be silicified, and probably was eroded from a carbonate matrix.  It
  consists of two pieces, nearly identical except for breakage.  Each is
  about an inch or an inch and a quarter long, cylindrical, and slightly
  compressed so that the cross section is oval or egg-shaped.    The long
  axis of the oval is also about an inch long.  Each piece appears to be
  solid, with no  natural openings to the inside, but a broken spot on one of
  them shows that it is hollow (not known if this is natural or a result of
  preservation).  The two pieces articulate tightly by means of apophyses
  projecting from each end (complementary sets are on the unarticulated
  surfaces, so it's a good bet that these two were once part of a longer
  series).  There are seven of these apophyses, which fit into seven
  corresponding recesses between the apophyses of the other piece.  Two pairs
  of these apophyses, the larger ones, are along the "long" sides of the
  oval, and at the "ends" of the oval cross-section are one and two smaller
  apophyses, respectively.  The margins of the apophyses, and the recesses
  into which they fit, are sculptured in a manner that reminds one of the
  sutures of ammonoids, or the joints between the bones of  a mammalian
  skull.  In its present condition, the sutures appear to play little role in
  the articulation of the pieces, but in life they probably did, producing a
  very tight junction.

  Best guess so far: decorticated phragmocone of some orthocone cephalopod.

  Why it probably isn't: no trace of siphuncle going through; the
  articulating surfaces when exposed are smooth and polished.

  Any help appreciated!

Best wishes, Bill
___________________
William A. Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943 USA
phone (804) 223-6172
FAX (804) 223-6374