144 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



from the group of Monof'hyllites sphcBrophylhis Mojs., but 

 that group has two or three auxiHary lobes and could not 

 have given rise to the simpler form without showing the 

 reduction in ontogeny. 



It was not found convenient to subdivide the adolescent 

 stage, for the reason that there was no marked change in 

 the surface or shape of the whorl. The septa become grad- 

 ually more complex until near the end of the third coil they 

 are transitional from Monophyllites to Lytoceras (PL XVII, 

 figs. 2 and 3; PL XVIII, fig. 13). This stage lasts for 

 about two revolutions, until near the end of the fourth 

 coil. 



Ephebic Stage. 



No line can be drawn between the adolescent and adult 

 stages in this species, for the only change is in the gradu- 

 ally increasing digitation of the septa. At three and three- 

 quarters coils, diameter of 7.50 mm., the septa are already 

 typical of Lytoceras (PL XVIII, fig. 14), while the form 

 has changed little, except that the whorl is slightly higher 

 in proportion to its width (PL XVII, fig. 4). The shell is 

 smooth except for the fine cross-strias and occasional varices. 

 The involution, always slight, becomes almost nothing, and 

 the whorl is somewhat squarer. PL XVII, fig. 6, shows 

 the spiral of the largest specimen obtained, diameter of 14.5 

 mm., with the relative height of whorl and width of umbili- 

 cus the same as in the adolescent period. The siphonal 

 collars now reach three-fourths of the distance forward 

 towards the next septum, longer than those described from 

 an}^ other ammonite. These certainly are separate organs 

 and not merely a prolongation of the septal walls. 



