Where did the companion mountain to Arsai Mons go?

1
Left: Viking Orbiter Mosaic, 1976                              Right: Resent Mars Map
 

References:
1 http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_10/geo_images_P-7/PlateP-7.jpeg
 or http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_10/GEO_PLATE_P-7.HTML