Classic USGS topographic maps can include a very large number of symbols so it is not practical to print a legend with all of them directly on the custom maps.
The full USGS legend is available at:
We suggest printing one of these for reference when examining a classic USGS topographic map.
Note that newer US Topo maps do include a key to symbols used on that particular quad. Aerial/Satellite Photos (and Hybrid TopoPhotos) do not include any legend.