PLATE XVII. 



Fig. 1. — Fragment of Actinostroma astroites, Rosen sp., from the Silurian (Upper 

 Oesel Group) of Kaugatoma-Pank, Esthonia, of the natural size. The specimen 

 shows the surface of one of the concentric laminas as exposed by fracture, and 

 exhibits the faintly-marked astrorbizae. (Page 143.) 



Fig. 2. — Tangential section of a specimen of Actinostroma astroites, Rosen, from 

 the Wenlock Limestone of Much Wenlock, enlarged 10 — 12 times. The minute 

 structure is better preserved in this than in any Russian specimen which I have 

 examined. 



Fig. .3. — Vertical section of the same, similarly enlarged, showing the dark 

 concentric lines of growth. 



Fig. 4. — Portion of the preceding section, enlarged about 24 times. 



Fig. 5. — Tangential section of another specimen of Actinostroma astroites, Rosen, 

 from the Wenlock Limestone of Much Wenlock, enlarged 10 — 12 times. The 

 specimen is traversed by numerous minute " Caunopora-tubes." 



Fig. 6. — Vertical section of the preceding, similarly enlarged. 



Fig. 7. — Vertical section of specimen of Actinostroma astroites, Rosen, sp., from 

 the Wenlock Limestone of Ironbridge, enlarged 10 — 12 times. The minute 

 structure of the skeleton, as in most examples of the species, is imperfectly pre- 

 served, and the coenosteum contains embedded in it numerous Spirorbes, arranged in 

 vertical rows as they became successively buried in the growing Stromatoporoid. 



Fig. 8. — Tangential section of Actinostroma fenestration, Nich., from the Middle 

 Devonian of Teignmouth, enlarged 10 — 12 times. (Page 146.) 



Fig. 9. — Vertical section of the same, similarly enlarged. The minute structure 

 of the skeleton is much obscured by crystallization and pressure. 



Fig. 10. — Vertical section of the type-specimen of Clathrodictyon vesiculosum, 

 Nich. and Mur., from the Clinton Formation, Yellow Springs, Ohio. Enlarged 

 10—12 times. (Page 147.) 



Fig. 11. — Tangential section of the same, similarly enlarged. 



Fig. 12. — Vertical section of a specimen of Clathrodictyon vesiculosum, Nich. and 

 Mur., from the Wenlock Limestone of Much Wenlock, enlarged 10 — 12 times. 

 (P. 147.) 



Fig. 13. — Tangential section of the same, similarly enlarged. 



Fig. 14. — A fragment of Clathrodictyon variolare, Rosen sp., of the natural size, 

 from the Silurian (" Raikiillsche Schichten ") between Saage and Ridaka, Esthonia. 

 The laminae in this specimen exhibit rounded " mamelons," but these are by no 

 means invariably present in this species. (Page 150.) 



