Another point of difference in this series of shields is the material spread over the wood to receive the incrusted stones. It must have solidified slowly to have allowed for the careful and laborious work of fixing the pieces in the plastic matrix. In the case of the matrix of gum, material could have been applied and would have hardened immediately, for by tests, even after the lapse of centuries, we have found it possible to loosen the bits by the application of heat to the stone; but in the case of the cement matrix this is not possible. We do not know what liquid was used to harden the cement, which is now quite friable. Relatively speaking, comparatively little turquois was used in this class of mosaics. Different shades of a soft light-gray stone was employed in the outer zones, while darker stones interspersed with turquois of varying shades, some almost white, and bits of lignite and obsidian, are found. A final unique feature may be noted in the shields on pls. XXVII and XXVIII. In the outer band will be observed the irregular inner edges of the two lines of stones which form the border. In pl. XXVII the space is filled in with a sprinkling of gritty, almost sand-like, bits of faded whitish-blue bastard turquois. It appears to have been spattered or sprinkled on while the matrix was very soft. In the case of the shield on pl. XXVIII, the material is somewhat coarser, and consists of rough bits of the same stone used in the broad light band within the outer border. These shields, so far as we are able to judge from the fragmentary sections, contained no pictures or figures as in the other type. In general execution they are inferior in workmanship, and less refined and finished in appearance. In fact, they constitute a quite distinct and, up to the present, unknown type of stone mosaic from Mexico. The first shield of this series (pl. XXIII) is in two fragments, a portion of one edge being missing. It has an average diameter of 15 inches, and a thickness of one-half to three-quarters of an inch, the mosaic disc rising one-eighth of an inch above the wood. The mosaic portion has a diameter of 10¼ inches. It appears to be a sun shield, judging by the eight pointers which radiate from the band surrounding the circular design in the center, which suggest a conventional representation of the sun. In the center the darker bits are obsidian. A portion of the matrix and of the mosaic incrustation has fallen off, and on the matrix near the upper left is a greasy patch which may be candle grease. The shield on pl. XXIV is the same size as the preceding one, 15 inches in diameter. It is also in two sections, and a larger section of the side is missing. This likewise is probably a sun shield, but it has no pointers. Where the outer band has fallen off toward the lower part can be seen a faint incised line which the artist placed there as a guide-line in following out the circular mosaic decoration. In the center the black bits are lignite.