140 CALlFORmA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



loid embryos, while the calcareous shell of the protoconch 

 is an ammonoid character pushed back by acceleration of 

 development until it occurs simultaneously with nautiloid 

 characters. No trace of a prosiphon could be seen on any 

 specimen, although some were nearly as transparent as 

 glass and would surely show this organ in transmitted light. 



Ananepionic. With the appearance of the first septum (PL 

 XVIII, fig. i) the animal becomes not merely a cephalopod, 

 but a chambered nautilian shell, and this is regarded as the 

 beginning of the larval period proper. As shown on PI. 

 XVI, figs. 3 and 4, this suture consists of a rather narrovv^, 

 rounded, siphonal saddle, flanked by a narrow, lateral lobe 

 and broader, lateral saddle. This stage corresponds to 

 some Silurian nautiloid, and while constant in all ammo- 

 noids is of equally short duration in all, lasting for only one 

 septum. 



Metanepionic. At the second septum the shell enters the 

 second larval stage, is no longer a nautiloid, but with the 

 development of a siphonal lobe becomes an ammonoid. 

 As Hyatt ^ has shown, this stage has in most ammonites, 

 especially the older forms, an undivided abdominal lobe 

 like that of the Nautilinidse of the Lower Devonian; but in 

 many later and more highly accelerated genera the stages 

 corresponding to the older goniatites are omitted. Thus in 

 JLytoceras alamedense (PI. XVIII, fig. 2, and PI. XVI, figs. 

 4 and 5) at the second septum the abdominal lobe is devel- 

 oped, but it is already divided by a siphonal saddle; this 

 Hyatt thinks corresponds to the Primordialidae of the Upper 

 Devonian; but all the older goniatites are retrosiphonate, 

 and only in extreme maturity of some late forms like Glyph- 

 ioceras do the siphonal collars point forwards. Now L. 

 alamedense is prosiphonate at the second septum, so that 

 this stage cannot correspond to the Primordialidas, but 

 rather to some of the late transitional goniatites, or to some 

 of the early simple ammonites of the Permian. The shape 

 and external septum resemble Glyphioceras, but that genus 



1 " Phylogeny of An Acquired Characteristic," pp. 414 and 415. 



