EXPLANATION OF THE FIGURES. 



FRONTISPIECE. The diagram opposite the title page is intended to 

 present, at one view, the distribution of the principal types of animals, 

 and the order of their successive appearance in the layers of the earth's 

 crust. The four Ages of Nature, mentioned at page 221, are represented 

 by four zones, of different shades, each of which is subdivided by circles, 

 indicating the number of formations of which they are composed. The 

 whole disk is divided by radiating lines into four segments, to include the 

 four great departments of the Animal Kingdom ; the Vertebrates, with 

 Man at their head, are placed in the upper compartment, the Articulates 

 at the left, the Mollusks at the right, and the Radiates below, as being 

 the lowest in rank. Each of these compartments is again subdivided to 

 include the different classes belonging to it, which are named at the outer 

 circle. At the centre is placed a figure to represent the primitive egg, 

 with its germinative vesicle and germinative dot, (278,) indicative of the 

 universal origin of all animals, and the epoch of life when all are appar- 

 ently alike, (275, 276.) Surrounding this, at the point from which each 

 department radiates, are placed the symbols of the several departments, 

 as explained on page 155. The zones are traversed by rays which repre- 

 sent the principal types of animals, and their origin and termination in- 

 dicates the age at which they first appeared or disappeared, all those which 

 reach the circumference being still in existence. The width of the ray in- 

 dicates the greater or less prevalence of the type at different geological 

 ages. Thus, in the class of Crustaceans, the Trilobites appear to com- 

 mence in the earliest strata, and to disappear with the carboniferous for- 

 mation. The Ammonites also appeared in the Silurian formation, and 

 did not become extinct before the deposition of the Cretaceous rocks. 

 The Belemnites appear in the lower Oolitic beds ; many forms commence 

 in the Tertiary ; a great number of types make their appearance only in 

 the Modern age ; while only a few have continued from the Silurian, 

 through every period to the present. Thus, the Crinoids were very nu- 

 merous in the Primary Age, and are but slightly developed in the Tertiary 

 and Modern Age. It is seen, at a glance, that the Animal Kingdom is 

 much more diversified in the later than in the earlier Ages. 



Below the circle is a section, intended to show more distinctly the rel- 

 ative position of the ten principal formations of stratified rocks (461) 

 composing the four great geological ages ; the numerals corresponding to 

 those on the ray leading to Man, in the circular figure. See also figure 154. 



