GEOLOGICAL BIOLOGY. 



sions of Brachiopods, in terms of the initiation of new genera, 

 is constructed (Fig. 58). 



Paleozic 



40 



20. 



I/ 



10 



Mesozoic Cenozoic 

 * ^\ ^ ' ^~~^ / *"*'" N 



Ordovloian Silurian Devonian Carboniferou^s, /fri. Jur. Cretaoeoii^ /Tertiary Qy. 



UA 



t-'L 



V 



r\ 



FIG. 58. Evolution curves of the Brachiopods. The spaces from left to right represent the suc- 

 cessive geological eras from the Cambrian to the Quaternary-Recent. The curves in the 

 upper part of the diagram are those of the Arthropomata, the lower curve that of the sub- 

 class Lyopomata. Starting from the horizontal base-line, elevation above this line expresses 

 the rate of evolution in terms of the number of new genera initiated in each division during 

 the era, the cross-lines representing 10, 20, 30, etc., new genera, respectively. The curves con- 

 nect the points so indicated for each group : a-a' = Arthropomata ; /-/' = Protremata ; b-b' 

 = Trullacea ; c-c' = Thecacea; /-/' = Telotremata ; h-h' = Helicopegmata ; n-n' = Ancylo- 

 brachia ; /-/' = Lyopomata. 



These facts expressed in numerical form are as follows: 



TABLE OF THE NEW GENERA INITIATED IN EACH GEOLOGICAL AGE, 

 GROUPED UNDER SUBCLASS, ORDERS, AND SUBORDERS (COMPILED FROM 

 SCHUCHERT'S LISTS). 



COS 



5 25 37 

 5 21 18 

 4 6 5 



i 15 13 



o 4 19 



? 2 3 



o i 15 



O I I 



Arthropomata, 220 Gen. 



Protremata, 



Trullacea, 



Thecacea, 



Telotremata, 



Rostracea, 



Helicopegmata, 52 



Ancylobrachia, 67 



81 

 19 

 62 

 139 

 20 



Its Interpretation. The two subclasses (if we call the 

 Brachiopods a class) Lyopomata and Arthropomata are 



