NicKLEs AND BASSLER.] DIAGNOSES OF GENERA. bl 



Semiopora Hall. 



Zoariiim a flattened, bifurcating frond, pi-oceedinj^ from a spread- 

 ing base; branches narrow, with parallel margins and a narrow non- 

 celluliferous marginal space; interzoa^cial spaces occupied by vesicular 

 tissue; apertures oval or subcircular, regularly disposed in parallel 

 longitudinal rows; apertures largest in marginal rows; between the 

 apertures in a longitudinal direction are the openings of two small 

 mesopores side by side. 



Genotype and only known species: Semiopora histigmata Hall. 

 Devonian. 



Ptilocella Simpson. 



A/oarium ensiform, with a pointed, striated base; apertures circular, 

 arranged in parallel, longitudinal rows, separated b}" ridges; margins 

 of frond striated, non-celluliferous. 



Genotype and onh" known species: PtilodictyajMrallelaYi^aW. Devo- 

 nian 



ACROGENIA Hall. 



Zoarium segmented, arising from cjdindrical rootlets; two segments 

 proceed from the truncated termination of the proceeding one; segments 

 obconical and striated at the base, gradually becoming flattened and 

 showing apertures; apertures largest in the marginal rows, with prom- 

 inent lunarium, in linear series separated by ridges. 



GenotA'pe and only known species: Acrogenia jyroliferaYi^W. Devo- 

 nian. 



Prismopora Hall. 



Zoarium ramose, of triangular, bifurcating or trifurcating branches, 

 sometimes anastomosing and forming clumps; branches with the sides 

 subequal, usually a little concave, edges sharp, sometimes serrated or 

 wavy; zocecia arise from mesothecse which radiate from the center to 

 the margins; apertures varying in arrangement, sometimes on the 

 summits of small papillte; structure otherwise as given for the family. 



Genotype: Prismopora triquetra Hall. Range, Devonian-Carbon- 

 iferous. Nine species. 



SCALARIPORA Hall, 



Zoarium with the faces of the triangular branches crossed bv salient 

 transverse ridges; otherwise like Prismopora. 



Genotype: Scalaripora scalar if argils Hall. Devonian. Five species. 



Glyptopora Ulrich. 



Zoarium a thin expansion, traversed on both surfaces by salient 

 ridges or crests, uniting so as to form larger or smaller cup-shaped 



