PLATE XVI. 



Page. 



Fig. 1. Ctoni,\tites crenistri.\ Phillips, showiiifr development of septa 68 



Lower Carboniferous, St. Louis-Chester stage, Mooretield, Ark. 

 a. First septum, ananepionic. 

 6. Second septum, metanepionic. 

 c Third septum, metanepionic. 



d. Fourth septum, iiaranepionie. 



e. Fifth septum, one-half revolution, diameter 0.74 mm., paranepionic. 



/. Eighth septum, one revolution, diameter 0.92 mm., ananeanic {Prionocems). 



g. Ananeanic septum, IJ revolutions, diameter 1.29 mm. 



/(. Septum at 1| whorls, diameter 1.37 mm.. 



('. Septum at 2.25 mm., 2 J revolutions, transitional from Prionocerns to Goniatilex. 



j. Septum at 15 mm., early adult. All figures magnified 20 times, except j, which is 

 twice enlarged. 

 Fig. 2. G astriocer.\s excelsum Meek 88 



Middle Coal Pleasures, Pope County, Ark. 

 .'(. Septa. 



6. Cross section of whorl. 

 Fig. 3. Muensteroceras parallelcm Hall 121 



Lower Carboniferous, Kinderhnok stage, Rockford, Ind.; septa natural size. 

 Figs. 4 and 5. Pericyclus blairi Miller and Gurley 60 



Lower Carboniferous, Kinderhook stage, Sedalia, Mo. 

 Figs. 6-8. Milleroceras parrishi Miller and Gurley 127 



LIpper Coal Measures, Kansas City, Mo. 

 Figs. 9-11. Neoicoceras elkhornexse Miller and Gurley 143 



Middle Coal Measures, Elkhorn Creek, Kentucky. 

 Figs. 12-14. Gastrioceras montgojieryense Miller and Gurley 95 



Upper Coal Measures, Montgomery County, 111. 

 Figs. 15-17. Schistoceras fultonexse Miller and Gurley 106 



LTpper Coal Measures, Fulton County, 111. 

 Fig. 18. Septa of Prolecanites lyoni Meek and Worthen 54 



Lower Carboniferous, Kinderhook stage, Rockford, Ind. 

 Fig. 19. Aganides rotatorius de Koninck 112 



Lower Carboniferous, Kinderhook stage, Rockford, Ind. 

 Fig. 20. Septa of Pericycli-s kochi Holzapfel 61 



Lower Carboniferous, Tournaisian stage, Erdbach, Germany. 

 Fig. 21. Septa of Popanoceras parkeri Heilprin 133 



Middle Coal Measures, Strawn formation. Wise County, Tex. 



Note. — Figs 4-17 loaned by the LTniversity of Chicago, from Bull. Illinois State Mus. Nat. His., 

 No. 11. 

 Fig. 18 is after Freeh, Die Steinkohlenformation, PI. XLVI, A, fig. 11. 

 Fig. 20 from Pal. Abhandl., Vol. V, PI. Ill, fig. 6. 

 Fig. 21 is from Proc. Acad. Sci. Phila., 1884, p. 53.) 



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