Geol.— Vol. I.] SMITH— LYTOCERAS AND PHYLLOCERAS. 1 43 



lobe becomes indented and the saddles somewhat irregular; 

 at diameter of 2.00 mm., one and seven-twelfths coils (fig. 

 9), this is more pronounced; at diameter of 2.03 mm., one 

 and two-thirds coils (fig. 10), the threefold division of the 

 first lateral lobe is seen; at two coils, diameter of 2.37 mm. 

 (fig. 11), the lobes and saddles, while still comparatively 

 simple, show all the main characters of the Lytoceratida?, 

 and as the sculpture ends here this may justly be taken as 

 the end of the larval stage. 



The paranepionic sculptured w^horl may then be divided 

 into two distinct periods: first, that part which has simple 

 goniatitic septa, lasting one-third of a coil; secondly, that 

 part with ammonitic septa, beginning at one and one-third 

 coils and lasting two-thirds of a revolution. This in turn 

 might be subdivided into several subordinate divisions based 

 on increasing complexity of the septa. No descendants of 

 Nannites with which this part of the larval stage could be 

 correlated are known. 



Adolescent Stages. 



When the sculpture dies away and the shell becomes 

 smooth again, the adolescent period may be said to have 

 begun. This happens at the end of the second whorl, and 

 was found on a number of specimens, to be constant at 

 diameter 2.3 mm. On PL XVI, figs. 11 and 12, is shown 

 the beginning of this stage, and the corresponding septa on 

 PL XVIII, fig. 12. On this specimen the first whorl is 

 smooth (metanepionic), the second sculptured, although 

 since the outer shell is broken off from most of the surface 

 the sculpture is indistinct and shows itself chiefly in undula- 

 tions rather than ribs on the cast. The characters of the 

 family Lytoceratidfie have now appeared, and this begin- 

 ning of the neanic period represents Mono^hyllites of the 

 Trias, and closely resembles the group of M. suessi Mojsis- 

 ovics, characterized by the presence of two lateral and one 

 auxiliary lobe. Mojsisovics ^ says that Lytoceras comes 



1 Detik. K. Akad. Wiss. Wien, Bd. LXIII, 1896, p. 95. 



