The most ancient seals are generally made of white marble, or shell, and sometimes also of lapis lazuli and serpentine. It is impossible to assign a definite or even an approximate date to the vast majority of cylinder-seals recovered from the ruined mounds of Mesopotamia, as most of them belonged to individuals otherwise unknown, but fortunately a number of seals have been brought to light which belong either to kings or officials whose date can be independently computed, and which therefore give us an illustration of the proficiency to which the art of engraving had been brought at the particular period in which the owners of the seals lived, and a comparison of the style of art exhibited on the otherwise undateable seals with those whose age has thus been fixed, makes it possible for us to assign them with some degree of certainty to the period to which they belong in the history of the art of seal-engraving. The interest of these small relics of the past is of course centred in the scenes depicted, which are very various, and which throw a flood of light upon the mythology, and elucidate many legendary uncertainties in the theological and religious conceptions of the Babylonians and Assyrians. Where a comparison with royal or official cylinder-seals of certain date is not feasible, the similarity between the style of art exhibited on the particular seal in question and that to which some sculptures of ancient patesis or kings conform afford us the necessary clue, while lastly, when both of these tests fail, if the seal bears an inscription, the character of the writing often enables us to place it in its right class. [Illustration: FIG. 49.]