PLATE XXVI 

 DIELASMA TURGIDUM Hall (Pages 412, 413) 



Figure 1. — The centronelliform stage of the loop; ventral view. 

 X9. 



Figure 2. — A later stage ; showing the resorption of the anterior por- 

 tion of the loop. X 6. 



Figure 3. — Early Dielasma stage, produced by further resorption of 

 the centronelloid loop. X 6. 



Figure 4. — Loop and crural plates of mature specimen, x 0- 



Figure 5. — Profile of the connecting band, x 6. 



Figure fi. — Side view of the loop, crm'a, and septum, x 6. 



St. Louis Group, Kentucky. 



ZYGOSPIRA KECUKVIROSTRIS Hall (Pages 413-416) 



Figure 7. — Centronelliform stage of the loop, x 12. 



Figure 8. — Profile of same. X 12. 



Figure fl. — A later stage; showing partial resorption of loop in 

 front and greater divergence of descending branches. X 12. 



Figure 10. — The same ; profile. X 12. 



Figure 11. — A little more advanced stage ; showing increased length 

 of connecting band. X 12. 



Specimens figures 7-11 are from the Trenton Shales, St. Paul, 

 Minnesota. 



Figure 12. — A looped stage, with broad, curved, descending branches 

 and more slender transverse band. X 6. 



Figure 13. — The same; profile. X 6. 



Figure 14. — A later stage; showing more slender loop. X 6. 



Figure 15. — A specimen showing curved, diverging, descending 

 branches, long transverse band, and two projections of the lamellje, which 

 are the beginning of the spiral cones. X 6. 



Figure 16. — A subsequent stage in which the lamellae are more ex- 

 tended and ventrally and inwardly curved. X 6. 



Figure 17. — The same ; profile. X 6. 



Figure 18. — A young individual in which there are about one and 

 one-half turns to each spiral. X 6. 



Figure 19. — The same ; profile, x 6. 



Figure 20. — The brachial sujiiports in a mature specimen. X 6. 



The specimens figures 12-20 are from the top of the Trenton, Frank- 

 fort, Kentucky. 



Figure 21. — The spirals and loop in a Canadian specimen of (Ana- 

 zyga =) Zygospira recur rirostris Hall. (After Davidson.) X 3. 



Figure 22. — The spirals and loop in {Hnllina ^=) Zygospira Saffordi 

 Winchell and Schuchert. Trenton, TenDess('l^ x 6. 



Figure 23. — The spirals and loop in (IlalUna ==) Zygonpira Nicolletti 

 Winchell and Schuchert. Trenton, Minnesota, x 6. 



Figure 24. — The spirals and loop in Catazyga Ileadi Billings. 

 (After Hall.) x2. 



Figure 2.5. — The spirals and loop in Zygospira moilesta Say. (After 

 Hall.) x 2f 



Figure 26. — The same. (After Davidson.) X -1^. 



