44 AMERICAN FOSSIL BRYOZOA. Lbull.173. 



Bactropora Hall. 



Zoarium simple or only slightly branched; lower extremity pointed; 

 otherwise as in Rhombopora. 



Genotype: Trematopora f granutriata Hall. Devonian, Mississip- 

 pian. Three species. 



Orthopora Hall. 



Zoarium ramose, solid; apertures in parallel longitudinal rows, with 

 ridges between; acanthopores present. 



Genotype: Trematojxjra regular is Hall. Silurian, Devonian. 

 Twenty-six species. 



ACANTHOCLEMA Hall. 



Zoarium slender, ramose; zooecia tubular, arising from a filiform 

 axis in the center of the branch; apertures oval, in diagonally inter- 

 secting series, or linear series separated by ridges; vestibules polygonal, 

 broadly sloping; acanthopores between the apertures. 



Genotype: Trematojpoixi alternata Hall. Devonian, Mississippian. 

 Six species. 



Nemataxis Hall. 



■ Zoarium slender, ramose; zooecia long, tubular, diverging obliquely 

 from a central filiform axis, near the surface bending abruptly; inter- 

 spaces solid; apertures oval, in linear series, separated by ridges, with 

 very thin, slightly elevated peristomes; on the surface at intervals 

 there are monticules destitute of cell apertures, which extend around 

 the branch, giving it an annulated appearance. 



Genotype: Nemataxis fibrosus Hall. Devonian. Two species. 



Tropidopora Hall. 



Zoarium slender, ramose; apertures in irregular longitudinal series, 

 separated by sinuous ridges, with very thin slightly elevated peristomes. 



Genotype and only known species: Tropidopoi'a nana Hall. 

 Devonian. 



Streblotrypa Ulrich. 



Zoarium ramose, slender, solid; zooecia long, tubular, diverging 

 from the center; inferior hemiseptum best developed, situated rather 

 far down; apertures regularly elliptical, with peristome, usually 

 arranged in longitudinal series; below the apertures there are from 

 one to twelve or more small pits, arranged, when numerous, in two or 

 three rows; occasionally very small acanthopores present. 



Genotype: Streblotrypa nicklesi Ulrich. Devonian, Carboniferous. 

 Fifteen species. 



