2s6 



GEOLOGICAL BIOLOGY. 



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

 is constructed (Fig. 58). 



Paleozic 



Mesozoic Cenozoic 



Carboniferous. /I'ri. Jiir. CrotaoeQii^ /Tertiary Qy.I^ 



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 dia.gram 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 lo, 20, 30, etc., new genera, respectively. The curves con- 

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

 = Trullacea ; c-c' = Thecacea; i-i' = Telotremata ; /i-A' = Helicopegmata ; n-n' — Ancylo- 

 brachia ; /-/' = Lyopomata. 



These facts expressed in numerical form are as follows: 



TABLE OF THE NEW GENERA INITIATED 

 GROUPED UNDER SUBCLASS, ORDERS, AND 

 SCHUCHERT'S LISTS). 



Arthropomata, 220 Gen. 



IN EACH GEOLOGICAL AGE, 

 SUBORDERS (COMPILED FROM 



Its Interpretation. 



Brachiopods a class) Lyopomata and Arthropomata are 



