30 AMERICAN FOSSIL BRYOZOA. [bull. 173. 



Mesotrypa Ulrich. 



Zoariuni hemispheric, conical, or discoidal, g'enerally free, with an 

 epitheca on the under surface ; zooecia prismatic or cylindrical, with 

 oblique and sometimes funnel-shaped diaphragms, which are probably 

 modihed cystiphragms; zooecia more or less separated by angular 

 mesopores, which become smaller with age, and are intersected b}^ 

 numerous diaphragms; acanthopores generally present, sometimes of 

 large size. 



Genotype: Dl^ilotrypa infida Ulrich. Ordovician, Silurian. Ten 

 described and seven new species. 



Family AMPLEXOPORID^ Ulrich. 



Zoarium usually ramose or discoidal, rarely bifoliate; zooecia simple, 

 prismatic tubes, with a well-marked divisional line (seen in tangential 

 sections as a fine black line) between adjoining tubes; diaphragms 

 present; mesopores practically absent, but small abortive cells some- 

 times occur among the larger ones in the monticules; acanthopores 

 generally abundant, but may be wanting. 



Amplexopora Ulrich. 



Zoarium ramose, discoidal, or massy; zooecia prismatic, with dia- 

 phragms; acanthopores alwa3\s present, variable in size and number. 



Genotype: Amfplexojpora cingulata Ulrich. Ordovician. Eight de- 

 scribed and four new species. 



MoNOTRYPELLA Ulrich. 



Like Amplexopora, but distinguished by the absence of acantho- 

 pores. 



Genotype: Monotrypella oequalis Ulrich. Range, Ordovician-De- 

 vonian. Nine species. 



Petalotrypa Ulrich. 



Zoarium Ijifoliate, of irregular compressed branches or simple 

 fronds; zooecia prismatic, arising from a strongly flexuous mesotheca; 

 apertures subcircular or polygonal; mesopore-like interspaces that do 

 not differ in their tabulation from the zooecia may occur; very small 

 acanthopores (?) occupy many of the angles of junction. 



Genotype: Petalotrypa compressa Ulrich. Devonian. Two species. 



Discotrypa Ulrich. 



Zoarium a thin, free, or parasitic circular expansion; surface smooth, 

 or with low, broad monticules; zooecia thin- walled, direct; apertures 

 hexagonal or rhomboidal, very regular in their arrangement, decreas- 

 ing in size from the centers of the monticules outward; neither meso- 

 pores nor acanthopores present. 



Genotype: Choitetes elegans Ulrich. Ordovician, Devonian. Two 

 species. 



