http://sharepoint.com/sites/dev3/_api/web/lists/getbytitle('Discussions')/items?$filter=substringof(%270x0120%27,ContentTypeId)

http://sharepoint.com/sites/dev3/_api/web/lists/getbytitle('Discussions')/items?$startswith(%270x0120%27,ContentTypeId)

http://sharepoint.com/sites/dev3/_api/web/GetFolderByServerRelativeUrl('/sites/dev3/lists/Discussions')/folders