  We report evidence for a striking difference between S0 galaxies in the local
field and in the Virgo Cluster. While field S0 galaxies have disks whose
surface-brightness profiles are roughly equally divided between the three main
types (Types I, II, and III: single-exponential, truncated, and antitruncated),
Virgo S0s appear to be entirely lacking in disk truncations. More specifically,
the fraction of truncations in S0 galaxies with M_B < -17 is 28% +7/-6% for the
field, versus 0% +4/-0% for the Virgo Cluster galaxies; the difference is
significant at the 99.7% level. The discrepancy is made up almost entirely by
Type I profiles, which are almost twice as frequent in the Virgo Cluster as
they are in the field.
  This suggests that S0 formation may be driven by different processes in
cluster and field environments, and that outer-disk effects can be useful tests
of S0 formation models.
