EXPLANATION OF THE FIGURES* 

 Plate IV. 



Fig. I. — Development of septa of Glyphioceras incisum Hyatt, 

 from the protoconch to the eighth septum, ^. 



Fig. 2. — Adolescent septum of G. incisum, 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. incisum, diameter 1.64 mm. {Prionoceras). 



Fig. 5. — Septum of G. incisum, diameter 2.25 mm., two and 

 three quarter whorls, transition from Priorioceras to Glyphioceras. 



Fig. 6. — Early adult septum of G. incisuyn, diameter 15 mm. 



Fig. 7. — Septa of Anarcestes siibnautilinus Sandberger (after 

 L. von Buch). 



Fig. 8. — Development of the septa of Tornoceras 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- 

 siensis, J. P. Smith (after J. P. Smith, Proc. Amer. Phil. Soc, vol. 

 XXV, No. 152, Plate XXIV, Fig. 4). 



Fig. II. — Septum of Gastrioceras branneri ]. P. Smith {loc. cit., 

 Plate XXIII, Fig. 6). 



Fig. 12. — Paralegoceras ioivense Meek and Worthen (after J. P. 

 Smith, loc. cit., Plate XIX. Fig. i). 



Fig. 13. — Septa of P. iowense (after J. P. Smith, loc. cit., Plate 

 XIX, Fig. lb). 



* The figures of G. incisum are from The Development of Glyphioceras 

 and the Phylogeny of the Glyphioceratidae, by J. P. Smith. Proc. Calif. 

 Acad. Sci., third series, vol. i. Geology, No. 3, Plate XV. The figures of 

 Tornoceras are from Branco, Palasontographica, vol. xxvii, Plate V, Fig. 7. 

 The septa of Pronorites cyclolobus are from Karpinsky, Ammoneen d. 

 Artinsk-Stufe, Plate I, Fig. 4 g-l. Drawings by Frances R. Smith. 



