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



continues in this for an entire revolution, to diameter 1.21 

 mm. (figs 5 and 6), during which time it is smooth, low- 

 whorled, broad, with narrow umbilicus. The forward 

 pointing siphonal collars are plainly visible in some speci- 

 mens, at first short, reaching only about one-sixth of the 

 distance toward the next septum. At one-third of a coil it 

 extends one-fifth forward, and at one and one-fourth coils 

 it extends one-third of the distance toward the next septum. 

 Paranepiovfc. At the end of the first whorl a deep con- 

 striction marking a temporary mouth of the shell makes its 

 appearance, and the surface ceases to be smooth, takes on 

 fine cross-ribs with fine striae of growth between them (PL 

 XVI, figs. 7 and 8), and at the same time the umbilicus 

 widens. The whorl narrows perceptibly, causing the pro- 

 toconch and first coil to bulge out, as shown on PI. XVI, 

 fig. 9 and PI. XVII, fig. i,^ The septa are still goniatitic, 

 and as long as that lasts, up to one and one-third coils, the 

 shell resembles Nannites hindostanus Diener; but shortly 

 after the end of the first coil at diameter of 1.58 mm., one 

 and one-twelfth coils, the two internal, lateral lobes begin 

 to coalesce (PL XVIII, fig. 6), showing a departure from 

 the Nannites stage. At one and one-third coils, diameter 

 1.81 mm., the two lobes have grown into one, and the 

 siphonal lobe becomes sHghtly notched (PL XVIII, fig. 7), 

 that is, begins to be ammonitic. The sculpture remains 

 just the same from the end of the first whorl to that of the 

 second (PL XVI, figs. 7 and 8, 9 and 10), but progression 

 is very apparent in the development of the septa. Hyatt ^ 

 says that the limits between the larval and adolescent stages 

 in the Lytoceratinge should be drawn where the septa 

 become ammonitic. To do this here would be artificial, 

 for the sculpture lasts until the end of the second whorl, 

 then stops suddenly, marking a distinct change regarded 

 as the end of the larval period. At diameter of 1.87 mm., 

 one and five-twelfths coils (PL XVIII, fig. 8), the lateral 



1 This stage resembles Pericyclus of the IvOwer Carboniferous, but probably there is 

 no real kinship. 

 • " Phylogeny of An Acquired Characteristic," p. 416. 



