53° 



HISTORICAL GEOLOGY 



were turreted (Turrilites, Fig. 493 H) ; one common form (Baculites, 

 Fig. 493 /) became straight like the Orthoceras ; others assumed forms 

 which seem to have been entirely a matter of accident, as is shown 



especially well in 

 a specimen from 

 Japan (Nipponites 

 mirabilis, Pig. 494 

 C). Many sug- 

 gestions have been 

 made to account 

 for the " death 

 contortions " of 

 the ammonites, 

 but none is satis- 

 factory. The one 

 most in favor is 

 that they mark 

 the senility of the 

 race. 



The descent of 

 ammonites from 

 goniatites (p. 459) 

 is shown in two 

 ways : (1) by com- 

 paring specimens 

 from successively 

 older formations 

 and noting the pro- 

 gressive changes, 

 and (2) by study- 

 ing the oldest and 

 youngest portions 

 of the shell of an 

 individual (Fig. 

 493 B). In these shells every stage in the growth of the individual is 

 preserved, so that if the shell of a full-grown ammonite is separated 

 along its septa from the apex to the living chamber and its sutures 

 studied, it is found that they increase in complexity — the first suture 

 or two made by the animal when young being simple, like those of the 

 .Silurian nautilus; then follow a few like those of the Devonian gonia- 



Fig. 494. — Cretaceous ammonites: A, Crioceras; B, Nosto- 

 ceras stantoni (Cretaceous). In this specimen the death of 

 the animal was probably caused by its own growth, as the 

 edge of the living chamber is almost in contact with the 

 lowest whorl of the spiral ; C, Restoration of Nipponites, a 

 remarkable genus from Japan. 



