156 NORTH AMERICAN INDEX FOSSILS. 



127. P. nebulosa (Hall). Lower Devonic. 

 A thin, leaf-like expansion from a thickened, narrow, striated 



base ; rows of cells increase rapidly by intercalation ; rounded 

 monticules, separated by about 3 mm. from each other, mark the 

 surface. 



Helderbergian of New York, etc. 



LXXIV. Escharopora Hall. 



Like Ptilodictya> but with the apertures in decussating series, 

 and surrounded by sloping hexagonal areas. Ord. 



128. E. falciformis (Nicholson). Ordovicic. 

 Falciform, narrow and long, apertures rhombic to oval, in diago- 

 nal curved series. 



Lorraine of the Cincinnati region and Tennessee. 



129. E. subrecta (Ulrich). (Fig. 208,/.) Ordovicic. 

 Elongate narrow, straight or curved, flattened and tapering 



below ; apertures oval, between regularly wavy ridges, which 

 regularly approach and diverge ; adult portion with a few meso- 

 pores, as in Streblotrypa. 



Black River of Minnesota, Iowa and Wisconsin. 



130. E. pavonia (d'Orbigny). Ordovicic. 

 Expanded, more or less irregular ; apertures oval to subcircular, 



with hexagonal vestibules and low, obscure monticules of slightly 

 larger cells ; arrangement of cells in curved lines as in E. falciformis. 

 Lorraine beds of the Cincinnati region and Tennessee. 



LXXV. Clathropora Hall. 



Anastomosing branches forming regular network with round or 

 oval fenestrules and pointed articulating base, apertures on both 

 sides usually subquadrate, arranged longitudinally. Sil.-Dev. 



131. C. frondosa Hall. (Fig. 210.) Siluric. 

 Flabellate or funnel-shaped fronds ; apertures rhomboidal or 



oblong quadrangular, opening obliquely upward. 



Clinton of Ohio; Niagara of Western New York and Ontario. 



LXXVJ. Arthropora Ulrich. 



Bushy, of numerous articulating equal segments spread in a 

 plane ; apertures on both sides, elliptical, surrounded by delicate 



