BRYOZOA. 371 



cystiphragms are developed just before opening at the surface. 

 Apertures subcircuiar or polygonal, the shape depending on the 

 thickness of the interspaces. Under favorable circumstances tan- 

 gential sections exhibit what appears to be large communica- 

 tion pores. 

 Type: H. curvata Ulrich. Range, Trenton to Niagara. 



PRASOPORA Nicholson and Ethridge. Zoaria forming hemi- 

 spheric, discoid, or, irregular masses, and, more rarely, thin ex- 

 pansions; usually free, with an epithecal covering on the lower 

 side. Zooecial tubes prismatic or cylindrical, thin-walled, largely 

 separated from each other by small angular mesopores, which 

 are generally quite inconspicuous at the surface. Acanthopores 

 usually present, but only occasionally either numerous or strong. 

 Cystiphragms in all the zooecial tubes. Diaphragms crowded in 

 the mesopores. 



Type: H. grayte Nich. and Ethr. Range, Trenton to Cincin- 

 nati. 



Family HETEROTRYPHLE n. fam. 



Zoaria frondescent, ramose or parasitic. Zooecia polygonal, 

 walls comparatively thin and more or less contiguous; rarely 

 sub-oval, with thick walls. Line of separation between adjoin- 

 ing zooecia more or less clearly defined. Mesopores varying 

 from very few to numerous. Acanthopores present, sometimes 

 of large size. Diaphragms numerous, usually horizontal. Cysti- 

 phragms wanting. 



HETEROTRYPA Nicholson. Zoaria frondescent, sub-ramose, or, 

 rarely, incrusting. Zooecial tubes prismatic, sometimes sub- 

 cylindrical. Apertures angular, sub-circular, or slightly peta- 

 loid. Walls moderately thin. Mesopores varying in number, 

 sometimes abundant, with illy defined walls. Acanthopores 

 small, generally numerous. Diaphragms well developed, usually 

 horizontal; sometimes a few may be concave or recurved. 



Type: H. frondosa d'Orb. sp. Cincinnati group. 



DEKAYIA Edwards and Haime. Ramose; branches cylindrical 

 or compressed. Zoo?cia polygonal, walls thin. Mesopores few. 



