286 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



Occurrence. — In America, common in most of the subdivisions of 

 the Black River (Decorah) shales in Minnesota and Iowa and in the 

 Clitambonites bed of the Lower Trenton of the same states. Appar- 

 ently rare in Russia, the only known specimen being from the Ortho- 

 ceras limestone (B3), near Baltischport, Esthonia. 



Plesiotype.— Cat. No. 57417, U.S.N.M. 



British Museum, thin section of the Russian type. 



Isssr %lir" — ■ 



Fig. 174.— Hemipheagma panderi. a, vertical section, X20, showing the large, thick semi- 

 diaphragms; 6, THE USUAL VIEW SEEN IN TANGENTIAL SECTION, X20; C, PORTION OF SAME, X40, SHOWING 

 STRUCTURE OF WALLS AND ACANTHOPORES. KUCKERS SHALE (02), BARON TOLL'S ESTATE, ESTHONIA. 



HEMIPHRAGMA PANDERI (Dybowski). 

 Plate 3, figs. 9-9c; text fig. 174, 



Orbiipora panderi Dybowski, Die Chaetetiden der Ostbaltisclien Silur-Formation, 



1877, p. 66, pi. 2, figs. 9-9c. 

 Cfr. Hemiphragma ottawaense (Foord), Contr. Micro-Pal. Cambro-Sil., 1883, p. 18, 



pi. 2, figs. 1-1/. 



Dybowski has given sufficient illustrations of this fine species (see 

 pi. 3, figs. 9-9c) to make its identification easy, and the additional 

 figures here presented are introduced to exhibit features of internal 

 structure not shown by the author of the species. As Dybowski's 

 figures show, the species is an undoubted typical HemipTiragma, and 

 to one acquainted with American forms its resemblance to Hemi- 

 pTiragma ottawaense (Foord), from the lowest Trenton of Canada, is 

 very striking. This resemblance is most marked in the surface 

 features of the zoarium and in the internal structure seen in vertical 

 sections. Tangential sections show, however, that the Russian spe- 

 cies has more angular zooecia, more numerous and more regularly 

 arranged acanthopores, and fewer mesopores or closed interspaces. 

 Thus, while the two are closely related, and possibly one is only a 



