为什么git://有效,但git @没有

为什么git://有效

$ git clone git://github.com/schacon/grit.git
Cloning into 'grit'...
...
Checking connectivity... done.

但是git @没有

$ git clone git@github.com:schacon/grit.git mygrit
Cloning into 'mygrit'...
Warning: Permanently added the RSA host key for IP address '192.30.252.129' to t
he list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

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