FOSSILS— THEIR NATURE AND INTERPRETATION. 85 



been since reported in the Cambrian for 7\mcrica alone by 

 Walcott; the Silurian has 54 genera, Devonian 39, Carbonif- 

 erous 34, Triassic 17, Jurassic 84, Cretaceous 1 10, Tertiary 

 125, and Recent 132 genera. 



Two Types of the Zoantharia indicated by the Two Maxima of 

 Genera in Separate Eras in the Time-scale. — In this series of 

 numbers of genera there are two maxima, one in the Silurian, 

 one in Recent time. This is explained by another fact : the 

 Order (Zoantharia) is divided, as above stated, into two bio- 

 logical groups, distinguished by a marked difference in the 

 numerical arrangement of radiating divisions of the body. 

 The first group, the Tetracoralla, has 81 genera; the second 

 group, the Hexacoralla, has 367 genera. With the exception 

 of a single genus, all the 81 genera of Tetracoralla are con- 

 fined to the Paleozoic. The Hexacoralla are mainly later 

 than the Paleozoic. 



These statistics for the Madreporaria, arranged in tabular 

 form, produce the following table (the figures in each column 

 opposite each family expressing the number of genera of the 

 family which made their first appearance in the geological 

 system corresponding to the letter at the top of the column) : 



TABLE OF THE NUMBERS OF GENERA OF MADREPORARIA 



MAKING THEIR FIRST APPEARANCE IN EACH GEO- 



LOGICAL SYSTEM, GROUPED. IN FAMILIES. 



Tetracoralla 81 genera 



Hexacoralla 367 genera 



1. Favositidae 



2. Poritidce 



3. Madreporidae 



4. Pocilloporidse 



5. Rupsammidse., 



6. Fungidaj 



7. Astrajida; 



8. Styloiiuridae 



9. Oculinidae 



10. Dasmidae 



11. Turbinolidae 



Total Madreporaria 448 genera 



O. 



D. Cr 



J. K. Ty. Q. R 



22 72 75 8 



Evolution Curve of a Group of Organisms. — These statistics 

 may be so arranged as to express in graphic form the rate of 

 generic differentiation. 



For this purpose a table is constructed, composed of a 

 series of ten perpendicular columns, each one, from left to 



