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 6. 



FIGURE 5. Profile of the connecting band, x 6. 



FIGURE 6. Side view of the loop, crura, and septum, x 6. 



St. Louis Group, Kentucky. 



ZYGOSPTRA RECURVIROSTRIS HALL (PAGES 413-416) 



FIGURE 7. Centronelliform stage of the loop. X 12. 



FIGURE 8. Profile of same. X 12. 



FIGURE 9. 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 trans verse band, and two projections of the lamellae, 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 supports 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 recurvirostris Hall. (After Davidson.) X 3. 



FIGURE 22. The spirals and loop in (Hallina =} Zygospira Saffordi 

 Winchell and Schuchert. Trenton, Tennessee. X 6. 



FIGURE 23. The spirals and loop in (Hallina =) Zygospira Nicolletti 

 Winchell and Schuchert. Trenton, Minnesota, x 6. 



FIGURE 24. The spirals and loop in Catazyga Headi Billings. 

 (After Hall.) X2. 



FIGURE 25. The spirals and loop in Zygospira modesta Say. (After 

 Hall.) x 2. 



FIGURE 26. The same. (After Davidson.) X 3. 



