40 



NORTH AMERICAN INDEX FOSSILS. 



2. A. fenestratum Nicholson. (Fig. 59.) Devonic. 

 Skeleton fibers coarser than preceding species — surface without 



the prominences (mamelons) ; astrorhizae well developed. 

 In Devonic limestones of Manitoba, etc. Also European. 



3. A. nodulatum (Nicholson). (Fig. 60.) Devonic. 

 In large expanding masses, with large sharp conical mamelons 



12 mm. apart and small astrorhizae. 

 Onondaga of Ohio. 



II. Clathrodictyon Nicholson. 

 Laminae a meshwork similar to preceding genus. Radial pillars 

 discontinuous, extending only from lamina to lamina. Astrorhizae 



Fig. 61. a, Clathrodictyon vesiculosum, X 8 ; b, Clathrodictyon striatellum, X&; 

 c, Strojnatoporella granulata, X 8. (All vertical sections.) 



present. Vertical section often appearing vesiculose. Siluric 

 (common)- Devonic (rare). 



4. C. vesiculosum Nicholson & Murie. (Fig. 61, a, 62, 63.) 

 Siluric. 



Laminar expansions covered by a concentric basal epitheca; 

 adult half a foot or more in diameter by an inch or more in great- 

 est thickness. No rounded 

 prominences; small astrorhizae 

 with centers from 3 to 5 mm. 

 apart. Nine to eleven lam nae 

 to 1 mm. 



In the Clinton and Niagara of 

 Ohio, Canada, New York and 

 Anticosti. Also European. 



5. C. striatellum d'Orbigny. 

 (Fig. 61, b.) Siluric. 

 Laminar or hemispheric with concentrically wrinkled epitheca. 



Surface undulating but no " mamelons,' generally exfoliating con- 



Fig. 62. Clathrodictyon vesiculosum,^ 

 60. (After Nicholson. ) 



