Square polygons appear as triangles

[Roberto Bianconi]

Hi,
I have a shp file that describes a grid plot. It is made of adjacent
square polygons and when viewed out of Mapserver it is displayed
correctly.
If I try to plot it with Mapserver, with something like:

    LAYER
       NAME grd
       STATUS DEFAULT
       TYPE POLYGON
       DATA /path_to_data/grd"
       CLASS
          EXPRESSION ([VALUE] > 1.e-5 and [VALUE] <= 1.e-4)
          COLOR   0 151 255
          OUTLINECOLOR 0 0 0
       END
       CLASS
          EXPRESSION ([VALUE] > 1.e-4 and [VALUE] <= 1.e-3)
          COLOR   0 223 255
          OUTLINECOLOR 0 0 0
       END
  END

I get the square polygons represented as triangles, with a line
connecting the lower right corner to the upper left corner (i.e. the
lower left vertex is not connected). If I plot this layer - as a test
-  as TYPE POINT and add a CLASS vector symbol and size, I can see the
4 corners of each polygon grid, each with the selected symbol.

Where am I wrong ? Any hint on how to get the correct behaviour?
Thank you.

Roberto