320 R. RUEDEMANN MODE OF LIFE OF PRIMITIVE CEPHALOPODS 



similar types. In later forms the growth of the apertural margin seems 

 to have become fairly uniform. 



Interior origin of Ankulation 



I should like to add here an observation giving a suggestion as to the 

 origin of the peculiar annulation observed in the Cycloceratidae. An 

 early annulated form of the Beekmantown of New York originally de- 

 scribed as Orthoceras cornu-oryx (now Orygocevas) Whitfield possesses 

 the annuli only as grooves on the inside of a rather thick shell that is 

 smooth on the outside, so that the casts only appear annulated. These 



Figure 8. — Fragment of Onjgoceras 

 cornu-oryx (Whitfield) 



Showing smooth exterior surface of shell 

 and casts of interior annulation. Natural 

 size. 



Figure 9. — Protocycloceras whiifiehli 

 (Ruedemann) 



Section of portion of conch, showing rela- 

 tion of annuli to septa. Natural size. 



grooves suggest that the annulation may have originated on the inside of 

 the shells by absorption from the necessity of gaining more room to place 

 the probably voluminous sexual products. The observation that the 

 annuli in the earlier forms, as notably the Fort Cassin species Protocy- 

 cloceras lamarki and P. luhitfielcU, exactly correspond in position and 

 number to the septa seems to corroborate this inference of the original 

 function of the annulation in these cephalopods. Later the annulation, 

 which could hardly have been of advantage in the movement of forms that 

 dragged their shells over the ground, appears to have persisted in the 

 Cycloceratidcie, because of its lending greater strength to the shells. 



