DEVELOPMENT OF BILOBITES 405 



with sub-equal triangular fissures, and the extent of the 

 dorsal and ventral beaks, are characters very much the same 

 as in Platystrophia biforata. 



These features are maintained until the neanic stage repre- 

 sented by figure 15, after which arrested hinge extension and 

 increasing areal growth in the ventral valve rapidly oblit- 

 erate the early characters, and in addition the growing loba- 

 tion of the valves emphasizes the expression of Bilobites. 



The genesis of the species is represented in figure 132, in 

 which it is shown that all these species are alike in their 

 development up to an early neanic period (b). B. Verneuilia- 

 nus (d) diverges at this point, progressively increasing its 

 variation from the normal direct growth, as exemplified in 

 B. bilobus (c). B. various (e) passes through all the bilobus 

 stages, and culminates in larger individuals, with less pro- 

 nounced lobation of the shell. 



The direct line of development is represented by B. bilobus, 

 and it is significant that this form also has the greatest geo- 

 logical and geographical distribution. Next, the divergent 

 and in direct line, typified by B. Verneuilianus and B. acuti- 

 lobus, is also widely distributed, but less so than the first. 

 Finally, the gerontic form, B. varicus, culminated and dis- 

 appeared within very narrow time and regional limits. 



