Thirteen square pillars topped by head-and-shoulder busts, so-called 'herms', all with a different beard. Sometimes they are called Philosophers or Emperors (by Max Beerbohm in Zuleika Dobson, 1911), or even the Twelve Apostles (by people who can't count to 13?).
The head were commissioned by the architect Christopher Wren in 1669. The present heads are the third set. The first set lasted 200 years, but by 1868 they were crumbling and new ones were erected. Between 1970 and 1972 the sculptor Michael Black, with two assistants, carved thirteen new heads for the Sheldonian, copying the originals by referring to the Loggan engraving; later he did another four for the Museum of the History of Science (see further here).
English sculptor of Oxford.
Sources & Information
Philip Opher, Oxford Sculpture (Heritage Tours Publications, 2009).