how to join several row values in one row in postgresql?

Question

How to turn :

=> SELECT pic.reference, spec.name FROM species spec NATURAL JOIN pictures pic
            WHERE spec.name = 'Bufo bufo';

 reference |   name    
-----------+-----------
 IM000213  | Bufo bufo
 IM000218  | Bufo bufo
 IM000222  | Bufo bufo
 IM000234  | Bufo bufo
(4 lignes)

into something like :

=> ???

 references                           |   name    
--------------------------------------+-----------
 IM000213,IM000218,IM000222,IM000234  | Bufo bufo
(1 ligne)

?

note : in postgresql 8.4