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



EXPLANATION OF PLATE XXVIIL 



Development of the Septa of Placenticeras pacificum. 



Fig. I. Placenticeras pacificum, septum at three and a sixth coils, diameter 

 8.50 mm., adolescent, about end of cosmoceran stage; 14/1. 



Fig. 2. P. pacificum, septum at three and a half coils, diameter 12 mm., 

 Hoplites stage ; 1 4/ 1 . 



Fig. 3. P. pacificum, septum at three and three-quarters coils, diameter 14.50 

 mm.; the septa already show placenticeran characters, although 

 the shell is still in the Hoplites stage; 9 times enlarged. 



Fig. 4. P. pacificum, septum at maturity; natural size. 



Fig. 5. P. pacifi.cuin, cross-section at four coils, adolescent, diameter 20.75 

 mm., showing inner whorls helmet-shaped, and the transition to 

 the compressed, flat-sided placenticeran whorl; 4 times enlarged. 



Fig. 6. Septum of P. californicum, adolescent, cosmoceran stage, two and 

 three-quarters coils, diameter 4.70 mm.; 13 times enlarged. 



Fig. 7. Early adult septum of Diaphorites vetulonius Fucini, for compari- 

 son. (After Fucini, Pal. Ital., 1896, Vol. II, PI. XXV, fig. S.) 



Fig. 8. Septum of Placenticeras guadaloupcc Roemer, for comparison. 

 (After Roemer, Kreidebildungen von Texas, PI. II, fig. i.) 



