At Susa the line of division between the prehistoric or neolithic age and the historical epoch is very clearly marked. The prehistoric stratum lies twenty-five metres below the surface of the mounds, and is divided by M. de Morgan and his fellow-workers into two periods. The first is distinguished by a fine thin pottery, with yellow paste, which is already made upon a wheel. It does not exceed from two to seven millimetres in thickness; it is polished, and decorated with black bands and various patterns in a brown colour produced by oxide of iron. The designs are not only geometric, but also represent animal and vegetable forms. Among them are rows of ostriches identical with those found on the painted prehistoric pottery of Egypt. Indeed, the explorers were especially struck by the resemblance of the pottery as a whole to that of Egypt in the prehistoric age, though it is difficult to see what connection there can have been between the two countries at so remote a date, and the curious similarity between the rows of birds depicted on the vases must remain for the present an archæological puzzle. There is also a certain amount of resemblance between the geometric pottery and that disinterred by M. Chantre at the early Assyrian colony at Kara Eyuk in Cappadocia, which will be discussed more fully in a later chapter.[43] Among the geometrical patterns of the Susian ware spherical forms are common; the herring-bone pattern is also met with, as well as a pattern like the Greek _sigma_. The under-part of the vases is often decorated, so also is the inside. A form of vase frequently found is the water-jar with a rounded foot; the goblet is another common shape. Sometimes the vases are supplied with four handles for suspension. This fine yellow pottery occurs not only at Susa, but also throughout Elam, but practically none of it has hitherto been discovered in Babylonia.[44] One cause of this is doubtless that in the alluvial plain of Babylonia a purely neolithic stratum, if it existed at all, would lie below the water-level. Maritime shells are met with as far north as the site of Babylon, showing that the Persian Gulf once extended thus far, and the water of the Euphrates still infiltrates through the soil.