218 SYSTEMATIC PALEONTOLOGY 



to large expansions 10 or more centimeters across. Some specimens are 

 of irregular shape, but almost always show their origin from the usual 

 hemispheric forms. Celluliferous side usually smooth, but sometimes 

 divided into polygonal, usually hexagonal, areas bounded by crestlike 

 elevations formed of mesopores. At the center of these areas is a similarly 

 elevated cluster of mesopores forming the macula. Zooecia thin-walled, 

 polygonal, averaging four in 2 mm. Mesopores few, restricted usually to 

 the macula and to the edges of the polygonal areas noted above. Acan- 

 thopores absent. Diaphragms placed at irregular intervals in the zooecial 

 tubes, but usually at distances varying between one and two tube diam- 

 eters. In the mesopores three diaphragms occur in a distance of their own 

 diameter. Walls exhibiting the minute granules seen in the typical 

 species of Diamdites. 



Occurrence. CHAMBERSBURG LIMESTONE (Echinospherites bed). 

 Pinesburg Station, Maryland. Middle Ordovician of Esthonia, Russia, 

 and Goodhue County, Minnesota (Prosser limestone). 



Collections. Maryland Geological Survey, U. S. National Museum. 



Family TREMATOPORIDAE 



Genus HEMIPHRAGMA Ulrich 



HEMIPHRAGMA IRRASUM (Ulrich) 

 Plate XLIV, Figs. 1-5 



Batostoma irrasa Ulrich, 1886, Fourteenth Ann. Kept. Geol. Nat. Hist. Surv. 



Minnesota, p. 94. 

 Hemiphragma irrasum Ulrich, 1893, Geol. Minnesota, vol. iii, p. 299, pi. xxiv, 



figs. 5-19. 

 Hemiphragma irrasum Bassler, 1911, Bull. U. S. Nat. Mus., No. 77, pp. 284 



286, text figs. 172, 173. 



Description. " Zoarium consisting of small, subcylindrical, frequently 

 and rather irregularly dividing branches, commonly 5 or 6 mm. in diam- 

 eter, but varying from 4 to 8 mm., the latter extreme probably only when 

 an extra layer of tubes has grown over the original branch. Monticules 

 wanting, but under fully matured conditions the surface is abundantly 

 spinulose. Zooecia with subangular apertures and thin walls when young, 



