In the 'man mongo' output, it mentions using mongod in place of mongo in the examples:

See this excerpt:

– excerpt from 'man mongo' follows –

EXAMPLES
mongo start the shell, connecting to the server at localhost:27017 and using the test database

mongod foo
start the shell using the foo database at localhost:27017

mongod 192.169.0.5/foo
start the shell using the foo database at 192.169.0.5:27017

mongod 192.169.0.5:9999/foo
start the shell using the foo database at 192.169.0.5:9999

mongod script1.js script2.js script3.js
run three scripts and exit

– end man page excerpt---