[plt-scheme] Dotted triple?

The other day in class, my professor typed (cdr '(a . b . c)) in 
DrScheme and got (a c) back. (car '(a . b . c)) gives back b. Just '(a . 
b . c) confirms that this is (b a c). I was just wondering why this is. 
Is it some peculiarity of PLT scheme or a bug or what? MIT Scheme and 
guile both complain about '(a . b . c). I was using R5RS as the language 
pack in DrScheme.

-- 
Chris Johnson
cjohnson at cs.wcu.edu
http://www.cjohnson.info