NicKLEs AND BASSLER.] DIAGNOSES OF GENERA. 31 



Family HETEROTEYPID^ TJlrich. 



Zoarium frondcscent, ramose, incrusting, or iiiussy; zooecia polj^g- 

 onal, Avitli moderately thin walls and diaphragms; walls of adjoining 

 zofficia coalescent; mesopores often present; acanthopores occur, some- 

 times of large size. 



Heterotrypa Nicholson. 



Zoarium erect, frondcscent, or ramose; zooecia with numerous dia- 

 phragms; acanthopores small, of one kind; mesopores variable in 

 number, generally abundant, sometimes wanting. 



Genotype: MonticuUpoi'a frondosa D'Orbigny. Ordovician, ^ De- 

 vonian. Eleven described and four new species. 



Dekayella Ulrich. 



Zoarium ramose; zocecia with numerous diaphragms; two sets of 

 acanthopores, large snd small; mesopores variable in number, gener- 

 ally numerous. 



Genotype: Dekayella ohsoura Ulrich. Ordovician. Nine described 

 and one new species. 



Dekayia Milne-Edwards and Haime. 



Zoarium erect, ramose; zooecia with few diaphragms; acanthopores 

 usually large; mesopores few, or wanting. 



Genotjq^e: Z>e^"«y/« cA§/^^a Milne-Edwards and Haime. Ordovician, 

 ? Devonian. Six species. 



Petigopora Ulrich. 



Zoarium forming small, circular, incrusting patches; zooecia direct, 

 with few or no diaphragms; acanthopores rather large, of one kind 

 only; mesopore.s wanting. 



Genotype: Petigopora gregariaVlvmh. Ordovician. Four described 

 and one new species. 



Leptotrypa Ulrich. 



Zoarium incrusting, often assuming cylindrical, discoidal, or other 

 forms; zooecial tubes polygonal, with thin walls, and few or no dia- 

 phragms; acanthopores very small, never abundant; no mesopores. 



Genotype: Leptotrypa minima Ulrich. Ordovician, ? Silurian, 

 % Devonian. Sixteen described and eight new species. 



Atactopora Ulrich. 



Zoarium forming thin, parasitic expansions, usually on species of 

 Orthoceras; zooecia direct, with few or no diaphragms; surface with 

 elevated, commonly elongated, subsolid monticules; acanthopores 

 small, very numerous, inflecting the walls, giving the apertures an 

 indented or petaloid aspect. 



Genotype: Atactopora MrsutaV)\\\it\\. Ordovician. Two described 

 and two new species. 



