48 AMERICAN FOSSIL BRYOZOA. [bull. 173. 



by numerous, irregular, thick-walled, tabulated mesopores, which are 

 also clustered at intervals into maculse; oritices circular, at the bottom 

 of the sloping vestibule. 



Genotype and only known species: Ileliotrypa hifolia Ulrich. 

 Mississippian. 



Family RHINIDICTYONID^ TJlrich. 



Zoarium bifoliate, continuous or jointed, consisting of compressed 

 branches or leaf -like expansions; occasionally trifoliate; zocecia sub- 

 quadrate, arranged longitudinally, inferior hemiseptum wanting; ori- 

 fices and apertures elliptical or subcircular, sometimes a little truncated 

 posteriorly; median tubuli between the median lamina and between 

 the longitudinal rows of zooecia; mesopores wanting, but vesicular 

 tissue often developed. 



Rhinidictya Ulrich. 



Zoarium ramose, consisting of narrow, compressed, bifurcating, 

 straight-edged branches with parallel margins, attached to foreign 

 bodies by a continuous expanded base; apertures in longitudinal series 

 between slightly elevated or flexuous ridges, carrying a crowded row 

 of small blunt spines; space immediately surrounding the apertures 

 sloping up to the summits of the ridges. 



Genotype: Rhinidictya nicholsoni Ulrich. Ordovician. Nineteen 

 species. 



EuRYDiCTYA Ulrich. 



Zoarium a broad, simple or irregularly divided, bifoliate expansion; 

 surface with more or less conspicuous, small, solid macular or monti- 

 cules; structure otherwise about as in Rhinidictya. 



Genotype: Eurydictya moyitifera Ulrich. Ordovician. Four spe- 

 cies. 



Pachydictya Ulrich. 



Zoarium ramose, of narrow bifurcating stipes, with parallel margins, 

 or irregular undulating fronds, with acute, nonporiferous margins; 

 surface with small maculoe, surrounded by apertures slightly larger 

 than the average; sometimes the marginal rows of apertures are also 

 slightly larger than the average; zooecia with thin walls, elliptical 

 or subquadrate in shape, separated from adjoining zooecia by small 

 vesicles; vestibules direct, walls thickened and appearing ringlike in 

 sections; spaces between vestibules traversed by one or more series of 

 minute tubuli; one or more diaphragms developed; apertures com- 

 monly elliptical; interspaces usually forming a peristome about the 

 apertures. 



Genotype: Pachydictya rohusta Ulrich. Ordovician, Silurian. 

 Twenty -five described and several new species. 



