404 



STUDIES IN EVOLUTION 



Observations. As shown in the ontogeny of B. various., 

 the generic stock was derived from a radicle having, in many 

 respects, the characters of the group represented by Platy- 



132 



<u 



I 



'b 



.2 



f 



O 



FIGURE 132. Genesis of Bilobites. 



a, nepionic stage. Ordovician type like Platystrophia biforata. X 4. 



b, nealogic period at which divergence begins. X 4. 



c, Bilobites bilobus. Niagara Horizon. X 2. 



d, Bilobites Verneuilianus, Niagara Horizon. X 2. 



e, Bilobites various. Lower Helderberg Horizon. X 2. 



strophia biforata. The general proportions of the nepionic 

 shell in B. various resemble it very closely. The length of 

 the hinge at this period, the high hinge areas in both valves, 



