DBMS_METADATA Valid Object Types

DBMS_METADATA takes object type parameters that do not always match the object type you'd find in DBA_OBJECTS. e.g. in the following, which gets DDL for a database link, notice that "DB_LINK", not "DATABASE LINK", is passed as the object type:

SELECT DBMS_METADATA.GET_DDL('DB_LINK','SCOTTS_LINK','SCOTT')
FROM DUAL;

To get the definitive object type list visit http://docs.oracle.com/cd/B19306_01/appdev.102/b14258/d_metada.htm#i998485 and navigate to "Table 57-18 Object Type Path Names for Heterogeneous Object Types"