NKKLES AND BASSLER.] DIAGNOSES OF GENERA. 45 



Hyphasmopora Etheridgc, Jun. 

 From the Carboniferous of Scotland. 



Family CHAINODICTYONID^. 

 Chainodictyon Foerste. 



Zoarium retiform, of inosculating branches, which are undulated 

 transversely on the reverse; zooecia elongate, conical, or subtubular; 

 apertures large, with thin interspaces in several rows, arranged in 

 acutely diagonall}" intersecting series. 



Genotype: Chainodictyon laxum Foerste. Carboniferous. Two 

 species. 



Family PTILODICTYONIDJE Zittel. 



Zoarium bifoliate, composed of two layers of zooecia, grown together 

 back to back, usually jointed, at least at the base, and forming leaf -like 

 expansions or compressed branching or inosculating stems; mesotheca 

 without median tubuli; zooecia usually have hemisepta and semiellip- 

 tical oritices; apertures usually ovate, surrounded either l)y a sloping 

 area or a ringlike peristome; vestibules separated by thick walls. 



Ptilodictya Lonsdale (Heterodictya Nicholson). 



Zoarium a simple, unbranched, lanceolate or falciform frond, narrow 

 or wide, which articulates with a small basal expansion; in the yoimg 

 condition the zoarium consists of longitudinally arranged, narrow, 

 ol)long-quadrate zo(jecia, new zooecia of different width and arrange- 

 ment being added subsequently on each side; walls of vestibules thick, 

 solid, and with a double row of minute dots. 



Genotype: I^lustralanceolataLionsdide. Ordovician, Silurian. Six- 

 teen species. 



EscHAROPORA Hall (NicHOLSONiA Waagen and Wentzel). 



Like Ptilodictya, but distinguished chiefly b}^ the arrangement of 

 the apertures, which are in decussating series, and by the sloping 

 hexagonal area which surrounds the apertures. 



Genotype: jKscharojforarecfaiiaW. Ordovician. Fourteen described, 

 and eleven new species. 



Clathropora Hall. 



Zoarium composed of anastomosing branches, forming a regular net- 

 work with round or oval fenestrules, with a pointed, articulating base; 

 apertures usually subquadrate, arranged longitudinally. 



Genotype: Clathro})m'a frondom Hall. Silurian, Devonian. Five 

 species. 



