  We present observations of the Class 0 protostar L1157-mm using the Combined
Array for Research in Millimeter-wave Astronomy (CARMA) in 3 mm dust continuum
and N2H+ line emission. In the N2H+ line, we detect a large-scale envelope
extended over a linear size of ~20,000AU flattened in the direction
perpendicular to the outflow. This N2H+ feature coincides with the outer
envelope seen in the 8 micron extinction by Looney et al. Meanwhile, the dust
continuum traces the compact, nearly spherical structure of the inner envelope,
where N2H+ becomes depleted. This highly flattened N2H+ envelope also shows
dynamical signatures consistent with gravitational infall in the inner region,
but a slow, solid-body rotation at large scales. This flattened structure is
not a rotationally supported circumstellar disk; instead, it resembles a
prestellar core both morphologically and kinematically, representing the early
phase of a Class 0 system. In this paper, we construct a simple model to
interpret both the dust continuum and N2H+ emission and suggest a possible
dynamical scenario for the overall properties of the envelope.
