EXPLANATION OF PLATE 11 



Figs. 1, 2 Triplophyllum terehratum (Hall) ; topotype ; American Museum of Natural His- 

 tory 4094/1 ; X 2.5 p. 38 



1, Transverse section, late ephebic stage ; dissepiments densely packed, septa long, thin, 

 many reaching axis ; f ossulae indistinct or not developed. 



2, Transverse section, late neanic or early ephebic stage ; twisting of septa and prominent 

 cardinal f ossula are characteristic. 



Figs. 3-6 Triplophyllites ida (Winchell) emend. Easton; x5 p. 41 



3, Transverse section, late ephebic stage ; hypotype ; septa radially arranged, thick, and 

 not numerous ; alar f ossulae absent ; after Grove. 



4, Transverse section, early ephebic stage ; hypotype ; septa remain in four groups, even 

 at this stage ; after Grove. 



5, Transverse section, reversed, middle ephebic stage ; paratype ; long thick counter sep- 

 tum, indistinct alar f ossulae, and swollen axial ends of some septa are characteristic ; 

 University of Michigan No. 23237. 



6, Transverse section, reversed, early neanic stage ; plesiotype ; radially arranged septa in 

 counter quadrants ; long cardinal and alar septa and pronounced alar pseudof ossulae 

 are noteworthy ; Illinois Geological Survey No. 3501. 



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