226 



CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Skr. 



Diameter . . 



Height of last 

 coil 



Height of last 

 coil from the 

 preceding 



Width of last 

 coil 



Involution . . . 



Width of umbil 

 icus .... 



The septa begin to resemble 

 Placenticeras. 



The fine sickle- 

 shaped ribs be- 

 come suddenly 

 coarse. 



The notched keels 

 give place to strong, 

 elongated nodes ,and 

 the central keel dis- 

 appears. 



3K coils. 



ram. 

 15.00 



8.00 



6.50 



4.00 

 1.50 



2.50 



6% coils. 



mm. 



172.00 



74.00 



52.00 



35.00 

 22.00 



35.00 



Conclusion and Summary of Results. 



The development of Placenticeras shows that it is possible, 

 in spite of dogmatic assertions to the contrary, to decipher 

 the race history of an animal in its individual ontogeny. 

 But the interpretation of ontogenic data is no simple problem 

 of mere comparison of growth stages with antecedent gen- 

 era; we must know the sources of error and apply the 

 necessary corrections. We must guard against unequal 

 acceleration, by which in the ontogeny of descendants 

 characters are cavised to occur together that belonged to 

 different geologic generations of ancestors. An example of 

 this is the central abdominal keel, that begins near the end 

 of the cosmoceran stage, when no Cosmoceras nor even 

 Hoplites ever had this; it is an adult character pushed back 

 further than the other placenticeran characters by unequal 

 acceleration. This inexact parallelism makes it impossible in 

 many cases to correlate exactly a growth stage with the 

 ancestral genus; otherwise it would be better to name the 



