294 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



The massive zoarium, extremely thin-walled polygonal zooecia, 

 absence of acanthopores, few mesopores, crenulated zocecial tube and 

 numerous semidiaphragms of H. subspliericum should suffice to dis- 

 tinguish it from associated as well as other massive bryozoans. 

 Externally Monotrypa jewensis is quite similar, but the vertical frac- 

 ture of H. subspliericum showing semidiaphragms will distinguish 

 them. A globose EemipJiragma in the lowest Trenton strata of the 

 United States is closely related to the form under discussion, but as it 

 remains undescribed, comparisons are unnecessary. 



Occurrence. — ^Abundant in the Wesenberg limestone (E) at Wesen- 

 berg, Esthonia, and vicinity. 



Cotypes.— Cat. No. 57423, U.S.N.M. 



Specimens and thin sections in the collection of the British Museum. 



HEMIPHRAGMA ROTUNDATUM, new species. 

 Text fig. 180. 



The type-specimen of this species is a rounded, subpyriform mass, 

 about 20 mm. long and 14 mm. in diameter, with a smooth, cellulif- 

 erous surface and inconspicuous maculae. The zooecial apertures 

 are direct, angular, thin-walled, and exhibit sharp acanthopores of 

 medium size at many of their junction angles. Five to six occur 

 in 2 mm. The mesopores are angular, usually small and few, seldom 



Fig. 180.— Hemipheagma rottjndatum. a, side view of the type-specimen, natural size; 6, tan- 

 gential SECTION OF THE TYPE, X20; C, A VERTICAL SECTION, X20, ILLUSTRATING THE TABULATION OF 

 BOTH ZOCECIA AND MESOPORES; d, A FEW ZOCECIA, X35. ORTHOCERAS LIMESTONE (B3), ISLAND OF 



RoGo, Esthonia. 



numerous enough to isolate the zooecia. Under a hand lens maculae 

 composed of larger zooecia and more numerous mesopores may be 

 observed at regular intervals. The zooecial apertures, when likewise 

 magnified, often exhibit the semidiaphragms at or near the surface 

 as a partition extending half way across the opening. 



Vertical sections exhibit two features in particular. First the 

 zocecial tubes are occupied by semidiaphragms, as noted above, 

 these appearing in thin sections as incomplete partitions projecting 



