162 



Cincinnati Society of Natural History. 



that their walls are thiu near the axis, and become much thickened as 

 the}' approach the surface. No diaphragms. In transverse sections 

 the cells radiate from the central axis, and appear as so many wedges 

 arranged around a central point. 



It is possible that this species is not congeneric with the Arthroclema 

 pulchella, Billings, upon which the genus was founded. But as I have 

 had no opportunity to examine specimens of that species, I have deemed 

 it prudent to refer my species provisionally to Mr. Billings' genus.* 



Formation and localit}^ : Quite abundant in middle Trenton strata, 

 at Lebanon, Tenn., where it is associated with numerous other Biyozoa. 



Ptilodictyonid^e. 



The family Ptilodictyoniolce as defined by Zittel (Handbuch der 

 Palseontologie, p. 603), comprises two distinct groups, which from the 

 distinguishing character may be termed u articulata" and "inarticu- 

 lata." Similar divisions have been made by Busk in both the Cyclo- 

 stomata and Cheilostomata, and it is interesting to note that such di- 

 visions can also be established in the proposed sub-order Treposto- 

 mata. T^he group "articulata" of the new sub-order contains, so far 

 as known, only the family Ptilodictyonida} as restricted {ante p. ), 

 and is characterized by a jointed zoarium. This character I have con- 

 sidered of sufficient importance to warrant the separation of the genera 

 having an unjointed zoarium, from those in which the zoarium is di- 

 vided into segments. Consequently, the family Stictoporidm has been 

 established for the reception of the genera having a continuous zoarium. 



Ptilodictya, Lonsdale. 



Heterodictya, Nicholson, Geo. Mag., vol. ii., n. s., 1875. 



Fronds simple or branched, springing from a pointed or wedge- 

 shaped, sub-solid, and finely striated base or articulating process, which 

 fitted loosely in the socket of the expanded and firmly attached base. 

 The free portion of the zoarium is two-edged, with the transverse sec- 

 tion acutely elliptical, with the surface either smooth, montiferous, or 

 marked b} r transverse ridges, and composed of two equal but distinct 

 sides ; each side is provided with a delicate epithecal membrane, from 

 which the cells arise to open on the two opposite faces of the frond. 

 Cells quadrate, rhomboidal, or hexagonal, and arranged in longitudinal 



* Since the above has been in press, I have been enabled, through the kindness of Mr. 

 S. A. Miller, to examine authentic specimens of Billings' species, and I am now convinced 

 that A. spiaiforme, though differing in many respects from the type species, is properly 

 referred to Arthroclema. 



