PLATE VI. 



Favositks Helderbeugi^. 



PaRe 8. 

 See Plates 4. 5. 

 Fig. 1. The upper surface of a subtui-binate foiin, which differs from the prevailing specimens in having 

 numerous and somewhat i-egulai-ly distributed larger cells, which are subcircular at their 

 apertures. 



Lower Helderberg g^-oup. Clarksmlle, N. Y. 

 Fig. 2. A specimen of unusually elongate or elevated turbinate form, presenting numerous larger cell 

 apertures, which are more conspicuous near the base. 

 Lower Helderberg group. Clarksvitle, N. Y. 

 Fig. 3. A vertical section of a specimen similar to fig. 3 of the preceding plate, in which the cells are fi-ee 

 from filling, and preserve, in an unusual degree, the transverse septa. 

 Lower Helderberg group. Clarksville, N. Y. 

 Fig. 4. A portion of a similar specimen enlarged, the septa being somewhat thickened from silicification. 

 Fig. 5. A vertical section where the cells are filled with siliceous matter, marked by the transverse septa 

 and showing the pores in the lateral walls, the mass being quite solid. 

 Lower Helderberg group. Clarksville, N. Y. 

 Rg. 6. A portion of specimen fig. 5, enlarged to show more distinctly the mural poi-es. 

 Fig. 7. A portion of a hemispheric mass where the columns, or filled cells, are slightly smaller, and the 

 open cells with more closely arranged septa than the prevailing forms of the species in New 

 York. 



Lower Helderbei-g group. Near Cumberland, Md. 

 Pig. 8. A hemispheric form, which is entirely silicified and solid, in which the cells are distinctly smaller 

 than the prevailing forms in New York. 



Lower Helderberg group. Near Cumberland, Md. 



