EXPLANATION OF THE FIGURES.* 
PiaTE IV. 
Fig. 1.—Development of septa of Glyphioceras incisum Hyatt, 
from the protoconch to the eighth septum, 22. 
Fig. 2.—Adolescent septum of G. izcisum, one and three fourth 
coils, diameter 1.29 mm., twenty times enlarged (Prionoceras 
stage). 
Fig. 3.—Septum of G. incisum, diameter 1.37 mm. (Prionoceras). 
Fig. 4.—Septum of G. imcisum, diameter 1.64 mm. (Prionoceras). 
Fig. 5.—Septum of G. incisum, diameter 2.25 mm., two and 
three quarter whorls, transition from Prionoceras to Glyphioceras. 
Fig. 6.—Early adult septum of G. imcisum, diameter 15 mm. 
Fig. 7.—Septa of Anarcestes subnautilinus Sandberger (after 
L. von Buch). 
Fig. 8.—Development of the septa of Zornoceras retrorsum 
Buch (after Branco). 
Fig. 9.—Development of the septa of Pronorites cyclolobus Phil- 
lips (after Karpinsky). 
Fig. 10.—Adult septum of Pronorites cyclolobus, variety arkan- 
stensis, J. P. Smith (after J. P. Smith, Proc. Amer. Phil. Soc., vol. 
xxv, No. 152, Plate XXIV, Fig. 4). 
Fig. 11.—Septum of Gastrioceras branneri J. P. Smith (loc. cit., 
Plate XXIII, Fig. 6). 
Fig. 12.—Paralegoceras towense Meek and Worthen (after J. P. 
Smith, Zc. cit., Plate XIX, Fig. 1). 
Fig. 13.—Septa of P. zowense (after J. P. Smith, Joc. cit., Plate 
XIX, Fig. 34). 
* The figures of G. zmctsum are from The Development of Glyphioceras 
and the Phylogeny of the Glyphioceratide, by J. P. Smith. Proc. Calif. 
Acad. Sci., third series, vol. i, Geology, No. 3, Plate XV. The figures of 
Tornoceras are from Branco, Palzontographica, vol. xxvii, Plate V, Fig. 7. 
The septa of Pronorites cyclolobus are from Karpinsky, Ammoneen d. 
Artinsk-Stufe, Plate I, Fig. 4 g-/. Drawings by Frances R. Smith. 
