To get the extent xmin bound of the entire layer:arcpy.Describe("Layer").extent.XMin
To get the extent xmin bound of each feature in the layer:searchRows = arcpy.da.SearchCursor("layer",["OID@","SHAPE@"])
for searchRow in searchRows:
oidValue = searchRow[0]
xminExt = searchRow[1].extent.XMin
print "OID = " + str(oidValue) + " has an xmin extent of " + str(xminExt)