Introduction
The basic structure of the ammonitella or embryonic shell of the Ammonoidea has been
well documented (Sandberger, 1851; Branco, 1879-1880, 1880-1881; Schindewolf,
1933; Erben 1960).  The ammonitella begins with a small egg-shaped or spherical initial
chamber (=protoconch) (for discussion of terms see Schindewolf, 1933; House, 1996;
and Landman et al., 1996; see Landman et al., 1996 for additional references) and
extends as a straight shaft or coiled tube (called the ammonitella coil by House, 1996:
168).  In most species the ammonitella extends approximately one whorl ending in a pri-
mary constriction (Landman et al., 1996; Klofak et al., 1999). Iiternally, the siphuncle
originates in the initial chainber as a rounded caecum and is attached to the shell wall by
a prosiphon (Landman, 1988: Figs. 1, 2 and Klofak et al., 1999: Figs. 1, 2).