34 AMERICAN FOSSIL BRYOZOA. [bull. 173. 



Thallostigma Hall. 



Family CONSTELLARIID^ Ulrich. 



Zoarium ramose, frondescent, laminar, or incrusting; zocBcia thin- 

 walled and prismatic in the immature region, with thicker walls and 

 subcylindrical when mature; apertures rounded, peristomes slightly 

 elevated; mesopores angular, abundant, commonly isolating the zooecia, 

 at intervals gathered into clusters, which are usually stellate, closed at 

 the surface, the closure Avith numerous minute perforations; true 

 acanthopores wanting, but granules often very abundant on the inter- 

 spaces; diaphragms in both zooecia and mesopores. 



CoNSTELLARiA Dana. 



Zoarium growing into erect, flattened branches or fronds from a 

 basal expansion which is attached to foreign bodies; surface with 

 depressed stellate macular, the spaces between the rays elevated and 

 occupied by two or three short rows or clusters of closely approxi- 

 mated apertures; mesopores aggregated into maculae, internally with 

 gradually crowding diaphragms. 



Genotype: Ceriopora constellata (Van Cleve) Dana. Ordovician. 

 Nine described and three new species. 



Stellipora Hall. 



Differs from Constellaria in its incrusting habit and in having only 

 mesopores in the interspaces between the raised zooecial clusters. 



Genotype and only known species: Stellipora antheloidea Hall. 

 Ordovician. 



NiCHOLSONELLA Ulrich. 



Zoarium a laminar expansion, sometimes giving ofl' flattened inter- 

 twining branches or fronds; interspaces often granose, interzooecial 

 spaces wide, filled with numerous mesopores, which haA^e thicker and 

 more numerous diaphragms than the zocecial tubes; with age the spaces 

 become filled up with a calcareous deposit, rendering the walls of the 

 mesopores unrecognizable. 



Genotype: Niclwlsonella ponderosa Ulrich. Ordovician. Five de- 

 scribed and nine new species. 



Idiotrypa Ulrich. 



Zoarium parasitic; zooecia and mesopores alike crossed by thick dia- 

 phragms at short and regular intervals, diaphragms apparently per- 

 forated by numerous minute foramina; mesopores surround the zooecia, 

 lumierous, irregular in shape, closed at the surface; walls with numer- 

 ous minute vertical tubuli or cells. 



Genotype and only known species: Idiotrypa parasitica Ulrich. 

 Silurian. 



