PROBLEM OF THE MONTICULIPOROIDEA I 59 



occurrence is sporadic, probably incidental to dying off of the 

 zooids, since these structures never occur within the cell opening 

 as tabulae, which they should if the zooids survived their forma- 

 tion. This species has no warts upon the walls and no " tubuli." 



Ptilodictya {^Escharopora) subrecta Ulr. is a simple bifoliate 

 frond, or if divided the branches do not diverge. The zoarium 

 is sword-shaped or feather-shaped, the point or lower end artic- 

 ulating to a crateriform, striated basal expansion. Specimens vary 

 from 10 to 50 mm long, I to I0 mm wide, and 0.5 to i.5 mm thick. 

 In fossils the basal expansion is separate from the frond, and 

 when living they could have united by a corneous interval or 

 the cortex alone. The frond began evidently very narrow and 

 grew by axial, apical cell increase, the growing end widening 

 more or less and again narrowing somewhat, according to growth 

 vigor. Marginal cell increase near, but below the apex also 

 occurred in some, but the older parts have a solid margin. The 

 cells have rather short prostrate thin-walled axial part and turn 

 more or less sharply into the peripheral direction, the walls here 

 thickening quickly. The cells are arranged, alternating in ver- 

 tical rows (Plate B, Fig 10). They are at first quadrangular, but 

 before reaching the peripheral region become hexagonal. Here 

 the solid interspace, or wall, thickens, so that oval or sub- 

 quadrate apertures remain, these being at the bottom of 

 calycals which are confluent in longitudinal rows, or oblique 

 rows in case of the cells that were added marginally. Sinuous, 

 long ridges bound the cell rows (see figure). In later growth thick- 

 ening, these ridges meet alternatingly at the sinuosities bounding 

 rhombic calycals and finally change to continuous peripheries, 

 the cell apertures becoming meanwhile somewhat larger, subhex- 

 agonal, except in the oldest zoarial part or stem where the fur- 

 rows remain. 



Thin sections show the cell walls completely amalgamated, 

 the sinuous longitudinal ridges alone being evident. No tabulae 

 are seen, except one or two hemisepta. But the exterior of 

 zoaria exceptionally show a few mesopore apertures, these being 



