166 THE PALEONTOLOGY OF MINNESOTA. 



[PtUodlctya. 



this condition seems to be permanent, the entire zoarium consists of longitudinally 

 arranged, narrow, oblong-quadrate zooecia. As growth proceeded new zooecia, both 

 wider and differently arranged, were added on each side. These lateral zooecia 

 may be arranged in oblique or transverse rows, so as to produce the "pinnate" or 

 "plumose" arrangement prevailing in the typical species, or they may form diag- 

 onally intersecting rows, with groups of large cells or subsolid spots raised at 

 regular intervals into monticules. Zooecial apertures subquadrate, rhomboidal, or 

 rounded, the shape ilepending largely on their arrangement. 



Both hemisepta usually well developed. Primitive cell, with thin walls, sub- 

 elongate, quadrangular, hexagonal, or lozenge-shaped, in contact at all sides. In 

 the vestibular or outer region, the walls are more or less thickened, solid, and with a 

 double row of exceedingly minute dots ; the latter rarely preserved and seen only in 

 tangential sections. No median tub uli. 



Type : P. lanceolata Goldfuss, sp. 



CLASSIFICATION OF SPECIES.* 



Section a; without monticules. 



p. lanceolata Goldf., Upper Silurian, Europe. 



P. expansa Hall (not Phcenopora expansa Hall and Whitefleld), Clinton group, Ohio. 



P. gigantia {Heterodictya gigantia Nicholson), Corniferous limestone, Canada. 



P. canadensis Billings, Hudson River group, Canada. 



P. flapellum Nicholson, Cincinnati group, Ohio. 



P. gladiola Billings, Hudson Eiver and Anticosti groups, Anticosti. 



P..{?) sulcata Billings, Anticosti group, Anticosti. 



P.(?) angusta Hall, Niagara group, Indiana. 



Section h; with monticules. 



p. magnifiea Miller and Dyer, Cincinnati group, Ohio, Illinois and Indiana. 



P. plumaria James (as figured by Ulrich) Cincinnati group, Ohio, Indiana and Illinois. ■ 



P. variabilis Ulrich, Cincinnati group, Ohio and Indiana. 



P. whiteavesi Ulrich, Hudson River group, Manitoba. 



P. nebulosa Hall, Lower Helderberg group, New York. 



No species of Ptilodictya, as here restricted and defined, have yet been brought 

 to my uoticie from Minnesota deposits, but it is not improbable that P. magnifiea 

 M. and D., occurs in the upper beds of the Hudson River group in the southern part 

 of the state, that species having been noticed as far to the northwest as Wilmington 

 and Savannah in Illinois. 



* A number of foreign species have been described as Ptilodictya, but in the absence of specimens I do not consider my- 

 self warranted to attempt their classification, 



