modifying legend in scatterplot matrix R

scatterplot.matrix(~ a + b + c,
           groups=treatment,

           legend.plot=T,
           )
legend("topright",c("fenced","unefenced"), fill=NULL,bty="o",cex=3)
--------------------------------------------------------------------

In my dataset, the variable "treatment" contains two levels that are coded "N"
and "Y" (for No and Yes) and appear in my "per default" legend that is located
in the buttomright of my 3x3 scatterplot. I would like to replace N by a list
of character: "unfenced" and Y by "fenced", as well as moving the legend in
another place in my graphic... is someone know how to do that with a
scatterplot?