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



EXPLANATION OF PLATE XXVIL 



Development of the Septa of Placenticeras pacificum. 



Fig. I. First septum, ananepionic, nautiloid stage; 20/1. 



Fig. 2. Septum at one-half coil, diameter 0.83 mm., metanepionic, glyphio- 

 ceran stage; 20/1. 



Fig. 3. Septum at one coil, diameter 1.16 mm., glyphioceran stage; 20/1. 



Fig. 4. Septum at one and one-twelfth coils, diameter i-32 mm., end of the 

 paranepionic or end of larval stage, Nannites stage; 20/1. 



Fig. 5. Septum at one and three-eighths coils, diameter 1.70 mm.; 20/1. 



Fig. 6. Septum at one and five-eighths coils, diameter 2.20 mm., 20/1. 



Fig. 7. Septum at one and three-quarters coils, diameter 2.40 mm., 20/1. 



Fig. 8. Septum at one and seven-eighths coils, diameter 2.70 mm., transi- 

 tional from ^^goceratidae to the Perisphinctin.Te; 20/1. 



Fig. 9. Septum at two coils, diameter 3 mm.; 20/1. 



Fig. 10. Septum at two and three-eighths coils, diameter 4. 10 mm., Cosuio- 

 ceras stage; 13/1. 



Fig. II. Septum at two and a half coils, diameter 4.50 mm., paraneanic, 

 Cosmoceras stage; 14/1. 



Fig. 12. Septum at two and seven-twelfths coils, diameter 5.50 mm., Cosmo- 

 ceras stage; 14/1. 



?%. 13. Septum at two and three-quarters coils, diameter 6.25 mm., para- 

 neanic, Cosmoceras stage; 14/1. 



