NiCKLES AND liAssLEK.] DIAGNOSES OF GENERA. 29 



PERONoroKA Nicholson. 



Zoarium bifoliate, throui^h l)niiK'hing of the fronds often forming 

 convoluted masses; surface smooth, usually with macular; zocecia with 

 rather thick walls, ring-like in transverse section, not inflected by the 

 acanthopores; cystiphragms abundant; apertures circular or subpolyg- 

 onal; mesopores and acanthopores varia})le in number. 



(ienoty])e: M(yntlenHp()i'(i froiuhmi Nicholson (not D'Orbigny) =■ Chm- 



tetex deelplenK Rominger. Ordovician. Two described and two new 



species. 



HoMOTRYPELLA Ulrich. 



Zoarium irregularly ramose or laminar; surface smooth, with small 

 macula?; zoo?cia rather thin- walled, cystiphragms usually confined to 

 the earlier part of the mature region and never present in the axial 

 region; apertures subcircular, sometimes faintly petaloid; mesopores 

 abundant, more or less completely separating the zooecia; acanthopores 

 numerous. 



Genotype: Ifomotrypella inMahiUs Ulrich. Ordovician, Silurian. 

 Eight described and five new species. 



HoMOTRYPA Ulrich. 



Zoarium frondescent or ramose; maculae or monticules of larger cell 

 apertures a characteristic feature; apertures often oblique; zooecia with 

 very thin or finely crenulated walls and remote diaphragms in imma- 

 ture region and cystiphragms, isolated or in series, confined to mature 

 region; mesopores few, in clusters; acanthopores generally developed. 



Genotype: Ilomotri/pa cu/'vataUlrich. Ordovician, Silurian. Eight- 

 een described and thirty new species. 



Prasopora Nicholson and Etheridge, Jun. 



Zoarium massy, usually free, with wrinkled epitheca on the under 

 surface; zooecia prismatic or cylindrical, thin-walled, withcy^stiphragms, 

 and generally surrounded by angular mesopores; acanthopores some- 

 times present, but rarely numerous or strong; diaphragms crowded 

 in mesopores. 



Genotype: Prasopora grayce Nicholson and Etheridge, Jun. Ordo- 

 vician. Fourteen described and six new species. 



Aspidopora Ulrich. 



Zoarium a thin expansion, sometimes of superposed layers, usually 

 free, with epitheca on under side; typically composed, according to 

 age, of from one to many subequal parts, each gently convex, with the 

 zocecia diminishing in size from center to margin; diaphragms wanting; 

 cystiphragms few in the zooecia; mesopores numerous, with close dia- 

 phragms; acanthopores usually present, always small. 



Genotj'pe: Aspidopora areo^ata\^\x\Q\i. Ordovician, Silurian. Eight 

 described and four new species. 



