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



shown on PL XX, figs i and 2, and the shell on PL XIX, 

 figs. 2 and 3. Even at this early stage the tendency to 

 form auxiliary lobes shows itself, in contradistinction to 

 Lytoceras. At one and one-sixth coils, diameter of 0.87 

 mm., the second internal lateral lobe has divided into two, 

 the outer of which soon gets above the umbilical margin. 

 This goes on repeating itself in later stages until a large 

 number of auxiliary internal and external lobes have been 

 formed. The first part of the paranepionic stage lasts up 

 to one and five-twelfths coils, but the umbilicus is always 

 wider than at corresponding stages of Lytoceras, and the 

 resemblance to Nannites not nearly so great. At this time 

 the lobes begin to be slightly ammonitic, and at one and 

 three-quarters coils, diameter of 1.42 mm. (PL XX, fig. 3), 

 are already decidedly so. The species has now parted 

 company with Lytoceras, for the lobes are too numerous, 

 and the sculptured stage of that genus is never reached. 



Towards the end of the second whorl this form has a 

 moderately wide umbilicus and ammonitic lobes, and then 

 seems to be transitional to that group of Monophyllites with 

 several lateral lobes ; but at the beginning of the third coil 

 the umbilicus becomes narrower, the last whorl higher and 

 more compressed, and the shell resembles Megapkyllites, a 

 Triassic genus which has been shown by Branco ^ to go 

 through in its development stages of growth somewhat like 

 those described for Phylloceras. This is the adolescent 

 period, and is shown on PL XIX, figs 6-8 and PL XX, 

 figs. 4 and 5. Shortly after this the septa become still more 

 complex, transitional to Phylloceras (PL XIX, fig. 9, and 

 PL XX, fig. 6), although the adolescent period does not 

 end abruptly, but at diameter of 7.00 mm. is still quite 

 apparent (PL XIX, fig. 10, and PL XX, fig. 7). At 10.00 

 mm. diameter the outHne and septa are already character- 

 istic of Phylloceras, and even of this particular species, as 

 shown on PL XIX, fig. 11. The adult septa shown on 

 PL XX, fig. 8, were traced from two photographs, at 



1 Beitr. Entwick, Foss. Cephal., PI. VII, fig. 4. 



