A Byzantine signet ring of the sixth or seventh century of our era, in the British Museum, shows the head of Christ, beneath which bending figures of two angels in profound adoration are depicted. Angel-figures almost exactly similar may be seen in Byzantine ivory carvings of this later period, the type evidently being one of those rigidly defined in the hieratic art of the school. With this ring were found coins of Heraclius (610–641), the Greek emperor in whose reign fell the death of Mohammed (June 8, 632) and the overthrow of the Sassanian Persian monarchy by the Mohammedans.[248] How important the possession of a royal seal-ring was considered to be, as proving the title of a successor, appears in the story that at the death-bed of Alexius Comnenus (1084–1118), Emperor of the East, when the son and rightful successor, John Comnenus, perceived that his mother Irene was working to exclude him from the throne and to seat thereon his blue-stocking sister Anna, he took off the imperial ring from the hand of his dying father and thus ensured for himself the title to the Eastern Empire.[249] Saint Bernard of Clairvaux (1091–1153), the enthusiastic preacher of the Second Crusade in 1147, excuses himself in some of his letters that he has failed to seal them, because he could not lay his hand on his signet. In a letter to Pope Eugene III, the saint complains that several spurious letters bearing his name have been circulated, sealed with a counterfeit seal; he also notifies the pontiff that from this time his letters will bear a new seal, on which will be his portrait and his name.[250] Well-to-do merchants of mediæval times, not entitled to armorial bearings, often had special individual marks or symbols engraved upon their signets. This custom obtained on the continent as well as in England, and allusion is made in the Old English poem of the fourteenth century, “Piers Plowman,” to “merchantes merkes ymedeled in glasse.”[251] Probably emblems of this kind came to have a certain association with the business which in many cases descended from father to son through a number of generations. A royal signet ring once believed to be that of Saint Louis (Louis IX, 1214–1270) and long preserved in the treasury of St. Denis, as an object of reverent care, is now in the Louvre Museum. The fact that the crescent is introduced as a symbol fails to connect the ring with the Crusader St. Louis, as this symbol was not used by the Saracens of his time, but was only adopted as a Mohammedan device after the Turks captured Constantinople, the crescent having been a recognised symbol in ancient times in Byzantium long before the city came to be called Constantinople.[252]