There remains to be considered the use of the bulbous mace-head and of the stone cylindrical seal, both of which are striking characteristics of the early Egyptian and Sumerian cultures. It is difficult to regard these classes of objects, and particularly the latter, as having been evolved independently in Egypt and by the Sumerians. In Babylonia the cylinder-seal is already highly developed when found on the earliest Sumerian sites, and it would appear that the Sumerian immigrants brought it with them into the country, along with their system of writing and the other elements of their comparatively advanced state of civilization. Whether they themselves had evolved it in their original home, or had obtained it from some other race with whom they came into contact before reaching the valley of the Euphrates, it is still impossible to say. The evidence from Susa has not yet thrown much light upon this point. While some stone seals and clay sealings have been found in the lowest stratum of the mound, they are not cylindrical but in the form of flat stamps. The cylindrical seal appears, however, to have been introduced at Susa at a comparatively early period, for examples are said to have been found in the group of strata representing the "Second Period," at a depth of from fifteen to twenty metres below the surface. The published material does not yet admit of any certain pronouncement with regard to the earliest history of the cylinder-seal and its migrations. In favour of the view that would regard it as an independent product of the early Egyptians, it may be noted that wood and not stone was the commonest material for cylinders in the earliest period.[16] But if the predynastic cylinder of Egypt is to be regarded as ultimately derived from Asia, the connection is to be set at a period anterior to the earliest Sumerian settlements that have yet been identified.