i 



Report of the State Geologist 85 



.Ptilodictya plumea, 



Ptilodictya pliimea, Hall. PalsBontoloecy of New York, vol. vi, p. 271, pi. 1x1, 



figs. 9-12, 1887. 



This species most nearly resembles P. retiformis, but may be 

 distinguished by the more nearly circular, more distant cell aper- 

 tures, and the convex, granulose, interapertural space ; from 

 P. scutulata it may be distinguished by the wider frond, the more 

 distant cell apertures, and the convex interapertural space ; from 

 P. falciformis, Nich., P. tenuis^ P. nehulosa and P. lirata, it may be 

 distinguished by the irregular disposition of the cell apertures^ ; 

 in those species the apertures being arranged in longitudinal rows. 



Formation and locality. — Hamilton group, Hamburg-on-the- 

 Lake, Erie county, N. Y. 



Tliamniscus pauciramus. 



Tliamniscus pauciramus, Hall. Trans. Albany Institute, vol. x, p. 197, 1881. 

 " " Eeport of State Geologist for 1883, p. 60, 



1884. 

 ** " ** Palaeontology of New York, vol. vi, p. 274, 



not figured, 1887. 



This species may be distinguished from Thamniscus variolata of 

 the Lower Helderberg group, by the much more slender branches, 

 less frequent bifurcations, and much more closely disposed cell 

 apertures ; from T. Cisseis by the less robust appearance of the 

 frond, the more slender branches and smaller cell apertures ; 

 from T. Nysa by the more slender branches, less frequent bifur- 

 cations^ and more closely disposed cell apertures. From T, mul- 

 tiramus of the Upper Helderberg group by the more slender 

 branches, comparatively less frequent bifurcations, smaller fronds 

 and their much more delicate appearance ; from T. nanus by the 

 less frequent bifurcations, the more uniform size of the branches, 

 and the much more closely disposed cell apertures. 



Formation and locality. — Hamilton group, Menteth's point, 

 west shore of Canandaigua lake, Ontario county, N. Y. 



