pg_locks relation cant be found in pg_class

[Mariel Cherkassky <mariel.cherkassky@xxxxxxxxx>]

Hey,

I'm trying to map an oid of relations in pg_lock to a relation name with the following query : 

select locktype,relname,database,mode,pid,granted from pg_locks,pg_class where locktype='relation' and pg_locks.relation=pg_class.oid;

However, I'm getting only relations of system catalogs and not other relations.. pg_locks contains rows for many relations, for example : 

postgres=# select relation,count(*) from pg_locks where locktype='relation' group by relation order by count desc;
 relation | count
----------+-------
    16531 |     4
   320587 |     3
    16535 |     3
   320585 |     3

but in the result of my first query I dont get the relation name. For example : 

postgres=# select count(*) from pg_class where oid in (16531,320587,16535);
 count
-------
     0

any idea ?