BRYOZOA. 235 



Homotrypa.l 



Genus HOMOTRYPA, Ulrich. 



Homotrypa, ULRICHI, 1882, Jour. Gin. Soc. Nat. Hist., vol. v, p. 240; 1890, Geol. Surv. 111., vol. viii, 



pp. 370 and 409; 1883, FOOKD, Contri. Micro-Pal, Can., pt. I, p. 9. 



Zoaria erect, generally ramose, at other times frondescent, with or without mon- 

 ticules. Zocecial tubes with very thin and finely crenulated walls and remote dia- 

 phragms in the axial region. Cystiphragms, isolated or in series, developed in the 

 peripheral region only. Apertures polygonal or subcircular, the shape depending 

 upon the thickness of the walls and the character of the interspaces. Mesopores 

 occasionally wanting, typically few and gathered into clusters ; in several aberrant 

 cases (e. g. H. separata), comparatively numerous and causing a greater or less separa- 

 tion of the zocecial walls. Acanthopores usually present, of varying sizes. What 

 appear to be large communication pores have been detected under favorable circum- 

 stances in several species. 



Type : H. curvata Ulrich, Hudson River group. 



This is the largest and one of the best characterized genera of the Trepostomata. 

 Adding the new forms here proposed we have a total of seventeen described species. 

 There are at least six more to be published from the Hudson River Rocks of Ohio 

 and Indiana alone, while as many more are known to me from various Lower Silu- 

 rian horizons in Tennessee, Illinois, Wisconsin and Minnesota. 



The affinities of the genus, as indicated by species like H. intercalates, are with 

 Homoirypella; others resemble Eridotrypa, while some again are not easily distin- 

 guised from erect forms of Monticulipara like M. arborea and M. molesta Nicholson. 

 In this connection it would be well to read the remarks on pp. 215 and 216. 



HOMOTRYPA MINNESOTENSIS Ulrich. 



PLATE XIX, FIGS. 1-9. 



Homotrypa minnesotensis ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 79. 



Zoarium ramose, branches subcylindrical, from 5 to 15 mm. in diameter, gener- 

 ally dividing dichotomously at long intervals, rarely inosculating. In the typical 

 form monticules are not developed, but the clusters of large cells are conspicuous. ' 

 In a later variety r commonly also of larger size than the typical form, these clusters 

 are often raised into prominent monticules, about 2.5 mm. from center to center. 

 Central part of these groups often appearing subsolid or with shallow mesopore-like 

 depressions in the interspaces. True mesopores wanting. Acanthopores exceedingly 

 small and few, observed only in thin sections. Ordinary zocecia angular, with thin 



