Historical clue formulations

• Things seen on a bookshelf — 2017-07-07
• Quills — 2015-09-13
• Cactus features — 2013-04-21
• Features of hedgehogs — 2011-05-26
• Bookcase lineup — 2008-03-16
• Places for book titles — 2006-03-14
• Library patrons read them — 2005-05-07
• Quills — 1999-10-04
• Vertebral columns — 1997-09-15
• Quills — 1997-01-05
• Porcupine quills — 1996-12-26
• Backbones — 1995-07-10
• Book parts — 1995-03-17
• Echidna features — 1994-09-17
• Places for titles — 1993-12-08