This cartoon by Paul Thomas from the Daily Express relates to news that the bones dug up last year under a parking lot in the English city of Leicester are the last mortal remains of England's King Richard III, the last English monarch to die in battle. See video report >>
The cartoon shows a group of forensic experts examining the King Richard's bones in the lab. The skull speaks saying, "A hearse, a hearse - my kingdom for a hearse." Which causes one of the experts to comment, "They are Richard III's bones ..."
To understand this joke, you have to know your Shakespeare. In Shakespeare's play Richard III, at the climax of the battle of Bosworth Field, Richard is unhorsed, and cries out, "A horse, a horse, my kingdom for a horse!" (one of the bard's best-known lines). Using a play on words, the cartoonist has changed "horse" to "hearse", because a hearse is a large car used to carry a dead body in a coffin at a funeral. Richard is telling us that he wants to be laid to rest, rather than laid out on a table in a lab.
In spoken English, Richard III is "Richard the third".