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 



CJ 



X 



X 



X 



C)|i 



X 



V 



• a 



O V 



P4 



FiGTRE 132. — Genesis of Bilohites. 

 a, nepionic stage. Ordovician type like Plati/slrophia hiforata. X 4. 

 h, nealofric period at which divergence begins. X 4- 



c, Bilohites bilobtis. Niagara Horizon. X 2. 



d, Dilobites Verneuilianus. Niagara Horizon. X 2. 



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



strophia hiforata. 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. 



