252 FOOT-NOTES TO EVOLUTION. 



in growth. All the pieces of whorls are preserved, but 

 often it is possible to have a complete series in one 

 specimen. The individuals representing stages of 

 growth are kept separate, in small glass tubes attached 

 to cards for labels, on which are noted the measure- 

 ments of the specimen, stage of growth, and such other 

 facts as are wanted for ready reference. 



The points to be noted in studying the development 

 of chambered cephalopods are : character of the proto- 

 conch or embryo shell ; position of the siphuncle, 

 whether internal, median, or external ; character and 

 direction of the siphonal collars, as on Plate V, Fig. 9, 

 where Tropites phocbus has the young siphonal collars 

 pointing backward, and the old ones pointing forward, 

 also change of the siphuncle from the internal to the 

 external margin of the whorl ; increasing lobation of 

 the septa with advancing growth; changes in the spiral 

 of the coil ; increasing involution, height of whorls, and 

 ornamentation of the outside shell. By observing care- 

 fully these changes in character, the successive stages 

 may be sharply distinguished from each other from the 

 beginning of the larval period to the old age of the 

 shell. 



No one species in its life history gives the entire 

 history of the race; the earlier forms do not get far 

 along in development, while the later ones hasten 

 through the earlier generic stages so rapidly that a 

 Jurassic or Cretaceous species often begins life where a 

 Paleozoic form in the same line of descent left off. We 

 are thus often forced to piece out the development his- 

 tory with successive species, using both the develop- 

 ment of the individual and the successive development 

 seen in the rocks. The writer has recently worked out 

 the development of Glyphioceras of the Carboniferous, 

 and Schloenbachia of the Cretaceous, in the same line of 



