On other graves beside those of the martyrs there are often found some little signs by which they could be easily recognized by the friends who might wish to visit them again.  Sometimes there is the impression of a seal upon the mortar; sometimes a ring or coin is left fastened into it; often a terra-cotta lamp is set in the cement at the head of the grave.  Touching, tender memorials of love and piety!  Few are left now in the opened catacombs, but here and there one may be seen in its original place,—­the visible sign of the sorrow and the faith of those who seventeen or eighteen centuries ago rested upon that support on which we rest to-day, and found it, in hardest trial, unfailing.

But the galleries of the catacombs are not wholly occupied with graves.  Now and then they open on either side into chambers (cubicula) of small dimension and of various form, scooped out of the rock, and furnished with graves around their sides,—­the burial-place arranged beforehand for some large family, or for certain persons buried with special honor.  Other openings in the rock are designed for chapels, in which the burial and other services of the Church were performed.  These, too, are of various sizes and forms; the largest of them would hold but a small number of persons;[F] but not unfrequently two stand opposite each other on the passage-way, as if one were for the men and the other for the women who should be present at the services.  Entering the chapel through a narrow door whose threshold is on a level with the path, we see at the opposite side a recess sunk in the rock, often semicircular, like the apsis of a church, and in this recess an arcosolium,—­which served at the same time as the grave of a martyr and as the altar of the little chapel.  It seems, indeed, as if in many cases the chapel had been formed not so much for the general purpose of holding religious service within the catacombs, as for that of celebrating worship over the remains of the martyr whose body had been transferred from its original grave to this new tomb.  It was thus that the custom, still prevalent in the Roman Church, of requiring that some relics shall be contained within an altar before it is held to be consecrated, probably began.  Perhaps it was with some reference to that portion of the Apocalypse in which St. John says, “I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held.  And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?  And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellow-servants also and their brethren that should be killed as they were should be fulfilled."[G] At any rate, these words must have dwelt in the memories of the Christians who came to worship God in the presence of the dead by whom they were surrounded in the catacombs.  But they knelt before the altar-tombs, not as before altars consecrated with relics of saints, but as before altars dedicated to God and connected with the memory of their own honored and beloved dead, whom he had called from them into his holy presence.