174 



Cincinnati Society of Natural History. 



the surface, the walls become thickened, and separated from each 

 other, the intervening space being narrow, and occupied b}' apparently 

 structureless sclerenchyma. This is the appearance when the tubes 

 are cut through their centers. When cut so as to pass along the side 

 of the tubes, the interstitial cells are brought to view. These are 

 crossed b} T numerous diaphragms, and remain open to near the surface^ 

 when they are filled b}' the interstitial membrane. This is usually tra- 

 versed by very slender, vertical dark streaks. The median laminae are 

 apparently amalgamated and poriferous, the minute foramina passing 

 very obliquely through them. 



In tangential sections, near the axis (PI. VIII., fig. 106), the tubes 

 are thin walled, sub-circular or broadly ovate, and usually in contact, 

 though sometimes nearly surrounded by the interstitial tubes. Nearer 

 the surface (PI. VIII., fig. 10c), the tube-walls are slightly thickened 

 and ring-like, while the interstitial spaces and " maculae 1 ' are profusely 

 dotted by minute tubuli (?spiniform tubuli). 



Of Pachydictya, beside P. robusta, I have one other large branching 

 species from the Niagara group of Kentucky, and a palmate and not 

 distinctly branched species from the Trenton of Tennessee and Min- 

 nesota, which are, without any doubt, congeneric. All have rather 

 large cells, and show more or less frequently the interstitial cells at the 

 surface. One of the principal characters of the genus is shown in 

 tangential sections. Namely, the cells are not separated into series by 

 distinct longitudinal lines, but the arrangement is more like that of 

 some of the M onticuliporidw (e. g. Prasopora newberryi, Nich.) In 

 all true species of Stictopora, such lines constitute a conspicuous 

 feature in tangential sections. 



Formation and locality : Collected by Prof. Satford in the lower 

 beds of the Trenton group, near Knoxville, Tenn. 



Phyllodictya frondosa, no v. gen. et sp. (Plate VIIL, figs. 11, Ma 



and 116). 



Phyllodictya^ gen. char, ante, p. 153. 



Zoarium growing from a somewhat expanded, sub-solid, sometimes 

 striated base, into erect, thin, simple and undulated expansions, which 

 are celluliferous on both sides ; the height of the fronds is always 

 less than two inches, while their thickness is never more than .08 inch, 

 and often not more than .04 inch. The edges of the fronds are pro- 

 vided with a distinct noc-poriferous margin. Cell apertures oblique, 

 according to the age of the zoarium, from T ^ to yi-g- of an inch in 

 diameter, usually more or less regularly arranged in intersecting diag- 

 onal series; in the perfect state the lower margin is prominent and 



