The position of the lion upon the fragment, represented with luxuriant mane and with head facing the spectator, and the vigour of the design as a whole combined with certain inequalities of treatment, have suggested a comparison with the lions upon the sculptured mace-head of Mesilim. The piece has, therefore, been assigned to the epoch of the earlier kings of Kish, anterior to the period of Ur-Ninâ.[50] It may perhaps belong to the rather later period of Ur-Ninâ's dynasty, but, even so, it suffices to indicate the excellence in design and draughtsmanship attained by the earlier Sumerians. In vigour and originality their representations of animals were unequalled by those of the later inhabitants of Babylonia and Assyria, until shortly before the close of the Assyrian empire. But the Sumerian artists only gradually acquired their skill, and on some of the engraved fragments recovered it is possible to trace an advance on earlier work. The designs in the accompanying blocks have been selected as illustrating, to some extent, the change which gradually took place in the treatment of animal forms by the Sumerians. [Illustration: Fig. 31.--Fig. 32.--Fig. 33. Three fragments of shell engraved with animal forms, which illustrate the growth of a naturalistic treatment in Sumerian design.--_Déc._, pl. 46, Nos. 4, 5, and 8.] Of the three designs, that on the left is engraved upon a convex piece of shell, thin as the shell of an egg; it represents a lion-headed eagle which has swooped down upon the back of a human-headed bull and is attacking him with mouth and claws. The subject resembles that found upon the most primitive Sumerian cylinder-seals, and its rough and angular treatment is sufficient indication of the very archaic character of the work. The central panel resembles in shape that of the lion and the bull.[51] The design represents a leaping ibex with flowering plants in the background, and the drawing is freer and less stiff than that of the animals on the silver vase of Entemena.[52] Some archaic characteristics may still be noted, such as the springing tufts of hair at the joints of the hind legs; but the general treatment of the subject marks a distinct advance upon the archaic conventions of the earlier fragment. The third design is that of a leaping kid, engraved upon a flat piece of shell and cut out for inlaying. Here the drawing is absolutely true to nature, and the artist has even noted the slight swelling of the head caused by the growing horns.