308 



BULLETIN 7*7, UNITED STATES NATIONAL MUSEUM. 



The maculae may be on a plane with the surface or may rise as rounded 

 or elongated monticules. These various developments of the maculae 

 are shown in Dybowski's figures and also on plate 11. 



Zooecia oval with a well-marked peristome, four in 2 mm. measuring 

 lengthwise, and shaped like the figure 8 because of a constriction in 

 the mid-length caused by the indentation of the walls by a pair of 

 large acanthopores (fig. 189 a). In some forms this regular arrange- 

 ment is somewhat obscured by the introduction of several additional 

 equally large acanthopores less regularly placed. These large acan- 

 thopores are ring-form with a conspicuous central open space or 

 lumen, and are in marked contrast with the second set of acan- 

 thopores, which consists of numerous smaller, more granular struc- 

 ures irregularly placed on the mesopore walls. The arrangement 

 and relative size of these two sets are illustrated in figure 189 a. 



4 



^m. 



Fig. 190.— Dittopora colliculata. a, small portion of a tangential section, X20, with the walls 



OF THE MESOPOEES VISIBLE. THE LARGE AND SMALL SET OF ACANTHOPORES ARE CLEARLY DEVELOPED; 

 6, ANOTHER TANGENTIAL SECTION, X20, WITH THE MESOPORES AND SMALL ACANTHOPORES OBLITERATED 

 BY THE INTERZOCECIAL TISSUE. THIS THIN SECTION WAS PREPARED FROM A SPECIMEN IDENTIFIED BY 

 DYBOWSEI AS TREMATOPORA PXJSTULIFERA; C, VERTICAL SECTION, X20, SHOWING THE OCCURRENCE OF 

 SEMIDIAPHRAGMS ONLY IN THE BEND TO THE MATURE ZONE. WASSALEM BEDS (D3), UXNOEM AND 



Wassalem, Esthonia. 



Mesopores closed at the surface, and so numerous as to isolate the 

 zooecia. 



The important features shown in tangential sections are the oval, 

 frequently constricted zooecia with thick, ring-like walls, the large 

 acanthopores with well defined central space, and the smaller, more 

 granular acanthopores irregularly placed in the inter-zooecial spaces. 

 Such sections prepared from near the surface of well developed speci- 

 mens show only a dense tissue penetrated by the smaller acanthopores 

 in the interzooecial spaces (fig. 190 &), but in deeper sections (fig. 190 a), 

 the outhnes of the mesopores are visible. 



On account of the thickness of the mature zone, it is difficult to 

 prepare vertical sections showing the full length of the zooecial tubes. 

 This difficulty was experienced by Dybowski, as is evidenced by his 

 figured vertical section (pi. 3, fig. 4b) of the type form of the spe- 



