Such stones, often made level at the top and bottom, and with a ring inserted in the upper surface, are not uncommon in the older churches of Rome, although they are very seldom noticed, as their significance is only known to a few experts.  One is placed in the centre of the middle nave of Santa Sabina, on the Aventine, on the top of a short spirally-fluted column of white marble, which marks the spot where St. Dominic, the founder of the order of the Dominicans, used to kneel down and pray.  It has received the name of Pietra di Paragone, or the Touchstone.  Another may be seen at the entrance of the church of Santa Pudenziana, on the Esquiline, supposed to have been built on the site of the house of the Roman senator Pudens, whose daughter, Pudentiana, St. Peter is said to have converted to Christianity.  A third exists among the extensive collection of relics belonging to the ten thousand three hundred martyrs whose remains, according to tradition, were deposited in the church of S. Prassede, at the beginning of the ninth century, by Paschal I. Two stones may be observed upon the gable wall immediately above the basins of holy water in the interior of the church of S. Nicolo in Carcere, near the Ghetto.  Two others are inserted in the wall of the Baptistery of St. John Lateran, between the vestibule and the octagonal area containing the so-called gigantic font in which Constantine was baptized.  A very interesting stone hangs suspended from the gilded iron grating which protects the crypt or confessional of St. Laurence, immediately underneath the high altar of the great Basilica of San Lorenzo beyond the Gate.  A stone still more remarkable, guarded by a strong iron grating, projects half its bulk from the wall on the right-hand side of the arch which divides the transept from the middle nave in the venerable church of Santa Maria in Trastevere.  Two other stones may be seen in the quaint old church of SS.  Cosma e Damiano at the south-eastern angle of the Roman Forum, composed of portions of three pagan temples.  They are inserted in the plain whitewashed walls on both sides of the circular arch through which you pass from the round vestibule into the interior of the church.  I have noticed similar stones in no less than twenty places besides those I have mentioned; and I am assured that they may be seen in many more churches.

It is very difficult to obtain any accurate or satisfactory information regarding these curious stones.  They go by the name of Lapides Martyrum, or Martyr-stones.  During the persecutions of the early Christians in Rome they are said to have been hung round the necks of those who were condemned to be drowned in the Tiber.  In the reign of the emperor Diocletian many martyrs perished in this way, and the stones by which they were sunk beneath the fatal waters, according to the popular idea, were afterwards found, and carefully preserved as holy relics in the churches in which they are now to be seen.  Beyond doubt they are