This concludes the presentation of the various forms which stand for the several periods of Table VIII. After an exhaustive study of these as found in Maya texts the writer has reached the following generalizations concerning them: 1. _Prevalence._ The periods in Initial Series are expressed far more frequently by head variants than by normal forms. The preponderance of the former over the latter in all Initial Series known is in the proportion of about 80 per cent of the total[48] against 12 per cent, the periods in the remaining 8 per cent being expressed by these two forms used side by side. In other words, four-fifths of all the Initial Series known have their periods expressed by head-variant glyphs. 2. _Antiquity._ Head-variant period glyphs seem to have been used very much earlier than the normal forms. Indeed, the first use of the former preceded the first use of the latter by about 300 years, while in Initial Series normal-form period glyphs do not occur until nearly 100 years later, or about 400 years after the first use of head variants for the same purpose. 3. _Variation._ Throughout the range of time covered by the Initial Series the normal forms for any given time-period differ but little from one another, all following very closely one fixed type. Although {74} nearly 200 years apart in point of time, the early form of the tun sign in figure 36, _a_, closely resembles the late form shown in _b_ of the same figure, as to its essentials. Or again, although 375 years apart, the early form of the katun sign in figure 36, _c_, is practically identical with the form in figure 36, d. Instances of this kind could be multiplied indefinitely, but the foregoing are sufficient to demonstrate that in so far as the normal-form period glyphs are concerned but little variation occurred from first to last. Similarly, it may be said, the head variants for any given period, while differing greatly in appearance at different epochs, retained, nevertheless, the same essential characteristic throughout. For example, although the uinal sign in figure 36, _e_, precedes the one in figure 36, _f_, by some 800 years, the same essential element--the large mouth curl--appears in both. Again, although 300 years separate the cycle signs shown in _g_ and _h_, figure 36, the essential characteristic of the early form (fig. 36, _g_), the hand, is still retained as the essential part of the late form (_h_). [Illustration: FIG. 36. Period glyphs, from widely separated sites and of different epochs, showing persistence of essential elements.]