282 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



has been assigned to the genus, and at least four undescribed branch- 

 ing forms have been discovered. In addition to this four American 

 species with the same structural features of incomplete diaphragms, 

 but with a massive form of growth, are awaiting description. The 

 genus Hemifhragma is therefore represented in America by no less 

 than thirteen species. The occurrence of ten species in the Middle 

 Ordovician of Russia and Sweden, seven of which are new, is an 

 interesting addition to our knowledge of the genus, and, as noted 

 before, emphasizes the value of incomplete diaphragms as a generic 

 feature. 



Genotype. — Batostoma irrasum Ulrich. Middle Ordovician (Black 

 River) of Minnesota and Iowa. 



Fig. 170.— Hemipheagma tenuimukale. a, the Russian example sectioned, nattjeal size; 6, tbetw 

 or tangential section theough the oldest part of the mature zone; c, vertical section, X20. 

 Wassalem beds (D3), Uxnorm, Esthonia. 



HEMIPHRAGMA TENXnMURALE Ulrich. 



Text figs. 170, 171. 



Hemifhragma tenuimurale Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, 

 pt. 1, 1893, p. 301, pi. 24, figs. 20-23. 



The following remarks, although based on the Russian specimens 

 of this species, apply also to the American types. 



Zoarium of small, irregularly dividing branches, 6 to 10 mm. in 

 diameter; surface smooth with inconspicuous maculae of zooecia 

 larger than usual; zooecial tubes polygonal, thin-walled; five to six 

 zooecia in 2 mm.; mesopores few, restricted abnost entirely to the 

 maculse; no acanthopores observed. 



