In short, most terminal emulators are limited in the control characters they can produce (for the simple reason that the original terminals that they are emulating were likewise limited).
So your terminal is very likely not producing anything other than . when you press C-. (and if that’s the case, that’s basically your answer, unless there is some way to configure the terminal to do otherwise).
(Any time you think Emacs is not acting on some input when running in a terminal, you should attempt to verify that Emacs is actually receiving the input you think it is.)
If your terminal is producing something other than ., then refer to the following Q&As for details on how to get Emacs to recognise the sequence:
- Binding M-<up> / M-<down> in Emacs 23.1.1
- Shift-Tab produces cryptic error in Emacs
- http://www.gnu.org/savannah-checkouts/gnu/emacs/manual/html_node/elisp/Translation-Keymaps.html
If you can run an xterm instead then you are probably in luck, due to some xterm-specific enhancements. Take a look at the following:
- Send “C-(” to Emacs in VT100/xterm terminal (Mac OS X’s Terminal)?
- http://www.dur.ac.uk/p.j.heslin/Software/Emacs/Download/xterm-extras.el
(I’m not sure that second link is relevant in this specific instance, but it seems worth including).
Some other related Q&As:
- How does one send S-RET to Emacs in a terminal?
- Why emacs confuse PageDown (<next>) key with M-[?