264 
PALAEONTOLOGY OF NEW YORK. 
sharp elevation; greatest thickness, without the elevation, from .50 to .75 
mm.; bifurcations on the specimens observed occur at intervals of from 15 
to 30 mm.; lateral branches very frequent, occurring usually at intervals of 
from 5 to 8 mm.; lateral branching and bifurcation sometimes occur together 
giving to the branch a trifurcating appearance; the branches usually diverge 
at an angle of about forty-five degrees. Cells tubular, cylindrical, gradually 
enlarging to the apertures; in the triangular branches they proceed from 
laminae which radiate from the center to each angle of the branch; 
in the flattened branches they proceed from the mesotheca, and are 
recumbent for about one-half their length, then abruptly turning and gener¬ 
ally opening directly outward (pi. lxii, fig. 18); at the center of the branch 
their direction is parallel to the axis of the frond, each succeeding row of 
cells becoming more and more divergent until frequently the marginal cells 
are nearly rectangular to the axis (pi. lxii, fig. 19). Intercellular tissue 
vesiculose; near the base the vesicles are large, smaller and more compactly 
disposed near the surface. Cell apertures disposed in longitudinal paral¬ 
lel rows, and very frequently in oblique ascending rows, from the middle of 
the branch ; usualty six or eight longitudinal rows on each face of the branch; 
the apertures of the central rows are minute, circular, with a diameter of 
about .12 mm., sixteen in the space of 5 mm.; marginal apertures sometimes 
circular, diameter .25 mm., at other times oval, length .30 mm., width two- 
thirds the length, ten in the space of 5 mm.; sometimes the apertures, with 
the exception of those of the marginal rows, are of the same size; at other 
times those of the central range are the smallest, those of each succeeding 
range being a little larger; on some fronds the apertures are distinctly 
papilliform. There is often a prominent, angular carina along the middle of the 
flattened branch, which has a height of from .50 to .75 mm., and a width at 
the base of .25 mm., generally straight, but sometimes a little sinuous; the 
sides of the triangular branches are concave and have no ridges or carina. 
Usually there are not more than eight ranges of apertures, and forms occur 
having only four ranges, while others have ten, but as all intermediate 
gradations occur, no specific distinction can be based upon this character; 
there is also considerable variation in the strength of the carina. 
