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



EXPLANATION OF THE FIGURES. 



Plate XVIII. 



Figs. 1-12 are thirty times enlarged ; fig. 13 is twenty times enlarged; and 



fig. 14 is fifteen times enlarged. 



Development of the septa of Lytoceras alamedense, sp. nov. 



First septum. 



Second septum. 



Seventh septum, one-half coil. 



At two-thirds of a coil, diam. 1.04 mm. 



At one coil, diam. 1.21 mm. 



At one and one-twelfth coils, diam. 1.58 mm. 



At one and one-third coils, diam. 1.81 mm. 



At one and five-twelfths coils, diam. 1.87 mm. 



At one and seven-twelfths coils, diam. 2.00 mm. 



At one and two-thirds coils, diam. 2.03 mm. 



At two coils, diam. 2.37 mm. 



At two and one-third coils, diam. 3.00 mm. 



At two and five-sixths coils, diam. 3.90 mm. 



At three and three-quarters coils, diam. 7.50 mm. 



