Geot..— Vol. I.] SMITH— PLAC ENTICE N AS. 215 



development of characters in young ammonites, it is usually 

 impossible to correlate them with certainty with any par- 

 ticular ancestral genus. And this difficulty increases 

 when we leave the larval stages, and are concerned with 

 later growth. 



The ananeanic stage begins at one and a-quarter coils, 

 diameter 1.50 mm., when the pointed lateral lobe develops 

 round prolongations on each side and becomes trifid (trias- 

 nidian) ; about the same time the lateral saddle becomes 

 indented, showing the first indication of digitation of the 

 septa. Sculpture appears on the shell here for the first 

 time, at first apparently merely a repetition of the constric- 

 tions and varices that began at the end of the first coil, then 

 developing faint but distinct ribs between the constrictions. 

 The septa at this stage are shown on PI, XXV^II, fig. 5, one 

 and three-eighths coils, and of the following dimensions : — 



mm. 



Diameter i . 70 



Height of last whorl o. 76 



Height of last coil from the preceding 0.61 



Width of last coil o . 95 



Involution 0.15 



Width of umbilicus o . 68 



This first adolescent stage resembles greatly Cymbites, 

 Dia^horites, and Pimelites, any one or all of which may 

 well have been ancestors of the ^goceran stock, although 

 it is impossible to speak with certainty or even probability, 

 especially since the relations of these genera to each other 

 are, as yet, only conjectural; the writer agrees with Buck- 

 man (1898), who regards Cymbiies as the radicle of the 

 asgoceran stock, but makes no mention of the other genera. 

 This stage will be referred to as the Cymbiies stage. 



On PL XXIV, figs. 10 and 11, is figured a specimen of 

 P.-pacificum in the Cymhites stage, one and seven-twelfths 

 coils and of the following dimensions: — 



mm. 



Diameter i . gg 



Height of last coil 0.78 



Height of last coil from the preceding 0.66 



Width of last coil 0.96 



Involution o. 12 



Width of umbilicus 0.75 



