EAELY PALEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 



269 



Internal characters. — In vertical sections the tubes have thin walls, are not entirely 

 vertical, and without diaphragms in the axial region. Near the surface they bend out- 

 ward rather abruptly when one and sometimes two diaphragms were in most cases 

 thrown across each tube. At the same time an abundant series of mesopores was 

 developed. These are crossed by from two to six diaphragms, the outer ones of which 

 are much thicker and separated by shorter intervals than the inner pair. * * * 

 The walls of the mesopores where two or more occupy an interspace are strongly 

 zigzag, in some instances appearing not unlike vesicular tissue. In tangential sections 

 the zooecial walls may be thin and occasionally even inflected by the acanthopores, 

 but as a rule they are ring-like, and generally completely separated from each other 



Fig. 158.— Trematopoea primigenia. a to c, three fragments, natural size; d, base of a zoarium, 



LIFE size; e, surface of a typical example, X9, showing the peristome; /, SURFACE OF ANOTHER 

 SPECIMEN, XlS, WITH NUMEROUS ACANTHOPORES AND OPEN MESOPORES; g, THE USUAL APPEARANCE 

 IN TANGENTIAL SECTION, X18; h, SEVERAL ZOCECIA OF THE SAME SECTION, X50; i, VERTICAL SECTION, 

 X18, THROUGH A BASAL EXPANSION ATTACHED TO A CRESTOID COLUMN. BLACK RlVEE (DECORAH) 



SHALE, Minneapolis, Minnesota. (After Ulrich.) 



by a series of unequal and irregularly shaped mesopores. The acanthopores are dis- 

 tinct, nearly uniform in size, usually attached to the outer side of the zocecial walls, 

 and number from one to three or four to each zocecium. 



Occurrence. — Common in the Khinidictya bed of the Black River 

 (Decorah) shale at various localities in Minnesota. Apparently rare 

 in the Wassalem beds (D3) at Uxnorm, Esthonia (Cat. No. 57372, 

 U.S.N.M.). 



Specimens from American localities are in the collections of the 

 British Museum. 



