Finally the fashion seems to have changed, and a more elegant form of coffin to have come into use. It was still of terra-cotta, but its surface was covered with a rich glaze originally blue but now mostly of a dark green. Here and there, on the parts shielded best from the atmosphere, the blue has preserved its colour. The general shape of these coffins is that of a shoe or slipper; the oval opening through which the body was introduced has a grooved edge for the adjustment of the lid. The small hole for the escape of gas is at the narrow end. This type seems to date from the last centuries of antiquity rather than from the time of the Chaldaean Empire; its examples are found close to the surface of the cemeteries, whence we may fairly conclude that they were the last accessions. It is still more significant that the images stamped upon the panels with which the lids are decorated have little to remind us of the bas-reliefs of Assyria and Chaldaea, and it is not until we turn to the medals of the Parthians and Sassanids that we find anything to which they can be readily compared.[444] In the cemeteries of Lower Chaldaea the various receptacles for human dust that we have described are heaped vertically one upon another, so that with the passage of time they have formed huge mounds covering vast spaces and rising conspicuously above the plain (see Fig. 167, letter c). Loftus tells us that at Warka he dug trenches between thirty and forty feet deep without reaching the lowest stratum of sepulchres. There was no apparent order in their arrangement. Sometimes brick divisions were found for a certain length, as if used to separate the tombs of one family from those of another. A layer of fine dust, spread evenly by the winds from the desert, separated the coffins. Terra-cotta cones inscribed with prayers had been thrown into the interstices. Sometimes, as at Mugheir, the mound thus formed is surmounted by a paved platform upon which open the drains that traverse the mass.[445] In most cases these mounds have been turned over in all their upper parts by the Arabs. It is probable that in ancient days each of these huge cemeteries had priests and superintendents told off to watch over them, to assign his place to each new comer, and to levy fees like those paid in our day to the mollahs attached to the Mosques of Nedjef and Kerbela. They guarded the integrity of the mound, and when it had reached the regulation height, caused it to be paved and finally closed.