BULLETIN OF THE LABORATORIES 
158 
circular, or slightly oval, arranged in longitudinal series, about eight 
in the length of 2 mm. Typically two interstitial cells are found at 
the extremities of the cells; in some specimens, however, additional 
interstitial cells appear along the sides of the cell, or even at the ex- 
tremities, so that the cells are sometimes completely surrounded by in- 
terstitial cells. The cell rows are separated by raised ridges, which 
appear like a narrow line running between the rows and closely pressed 
by the cells and interstitial cells on either side; or they become quite 
broad and poorly defined, bearing on their surface all the laterally sit- 
uated interstitial cells, the mouths of which are apt to be of an elon- 
gated rather than circular shape; whenever the interstitial cells become 
very numerous these separating ridges are apt to disappear entirely in 
a disordered mass of cells and interstitial cells. All these changes 
may appear in one and the same species. At the summit of the 
tubercles one or more cells are apt to be suppressed ; their places are 
then taken by interstitial cells. The edges of the frond are acute ; no 
non-poriferous margin has been observed, but considering the nature 
of the growth of the species, this is rather to be expected. Specimens 
are usually found divided in the middle, the epithecal laminae being 
exposed, and the celluliferous faces being adherent to the rock 
in which the specimens were imbedded. The epithecal mem- 
brane is smooth, or wrinkled transversely in curves, which 
to some degree mark various stages of growth, and conform 
therefore with the general outline of the flabellate frond. Through 
the epithecal membrane longitudinal lines may be seen, which are the 
laminae that separate the cell rows ; between these are diagonal lines 
dividing the surface into small subimbricating rhombs ; sometimes 
short transverse lines intersect the diagonal lines of the rhomb. From 
the epithecal laminae the cells diverge to the opposite faces of the 
frond, making an angle of 65° or more with the epithecal laminae. 
Fronds vary in size; one of the largest specimens seen being 100 
mm. broad and 130 mm. long. Eight cells are found in the length of 
2 mm ; eight or nine rows occupy the same distance measured trans- 
versely. The thickness varies from a thin expansion to a frond 2 mm. 
thick, rarely attaining a thickness of even 5 mm. 
The figure accompanying the original description of Ptilodictya 
expansa^ Hall and Whitfield, belongs to this species and the description 
of that species shows that specimens of this species were by mistake 
used in the determination of its characters. This species is evidently 
