pg_locks relation cant be found in pg_class

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 ?