The Doheny Reading Room ceiling contains marks of 14 early printers who were important in the history of book-making.
Read from left to right they are the marks (symbols) used by Fust and Schaeffer, Juan de Rosenbach, Juan Froben, Richard Pynson, the St. Albans Printers, Guy Marchant, Jean du Pre, William Caxton, J. Treschel, Aldus Manutius, Juan de Colonia and Nicolas Jenson, John Siberch, Berthold Rimbolt, and again Fust and Schaeffer and Juan de Rosenbach. Above the desk on the side wall the fifth mark is that of John Scolar. Because of William Caxton's importance in English printing, his mark is prominently located above the clock.
For more detail, see Ruth Wallach's online guide at:
http://www.publicartinla.com/USCArt/Doheny/printers_marks.html
and
The History of the Doheny Library.