EVOLUTION OF FOSSIL CEPHALOPODA. 



235 



their minute counterparts in youthful stages of later 

 forms; in fact, he could often furnish just as exact a 

 description of the form as if he had the adult genus 

 before him. 



NOMENCLATURE OF STAGES OF GROWTH. 



In order to correlate ontogenetic stages with the 

 generic changes seen in the development of the race it 

 is necessary to have an exact scientific nomenclature. 

 The most satisfactory is that given by Professor Hyatt 

 in Phylogeny of an Acquired Characteristic.* 



TABLE OF ONTOGENETIC STAGES. 



With the embryonic stage the paleontologist can do 

 nothing, except the very last substage or phylembryo, 

 when the Mollusca, Brachiopoda, and other groups be- 

 gin to secrete their shells ; but all the later stages are 

 easily accessible in well-preserved material. 



The best example of correlation of ontogenetic 



* Proc. Am. Phil. Soc., vol. xxxii, No. 143, pp. 391 and 397. 



