236 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



tially the same structure in the American form Monotrypa cumulata, 

 and its identity with D. petropolitana is further evidenced by direct 

 comparison of specimens of the two. 



Dianulites petropolitana may be defined briefly as follows : 

 Zoarium massive, usually hemispheric with a slightly concave, 

 epithecated base, and about 2.5 cm. 'wide, but ranging from this to 

 large expansions 10 or more cm. across. Some specimens are of irreg- 

 ular shape but almost always show their origin from the usual hemi- 

 spheric forms. Celluliferous side usually smooth, but sometimes 

 divided into polygonal, usually hexagonal, areas bounded by crest- 

 like elevations formed of mesopores. At the center of these areas is a 

 similarly elevated cluster of mesopores forming the macula. Zooecia 

 thin-walled, polygonal, averaging four in 2 mm. Mesopores few, 



d 



Fig. 132. — Dianulites peteopolitana. Ulrich's views of Monotrypa (Chaetexes) cumulata, intro- 

 duced FOR COMPARISON. a AND C, TANGENTIAL SECTION, X9, AND A SMALL PORTION, X35, SHO"WING 

 WALL structure; 6, VERTICAL SECTION, X9. LOWER PART OF THE TRENTON LIMESTONE, GOODHUE 



County, Minnesota. 



restricted usually to the macula and to the edges of the polygonal 

 areas noted above. Acanthopores absent. Diaphragms placed at 

 irregular intervals in the zooecial tubes but usually at distances vary- 

 ing between one and two tube diameters. In the mesopores three 

 diaphragms occur in a distance of their own diameter. Walls exhib- 

 iting the minute granules seen in the typical species of Dianulites. 



In figure 129 a the usual structure seen in thin sections is illustrated, 

 although it is difficult to give in a drawing a correct impression of the 

 peculiar granular waU structure. In these figures it will be noted 

 that mesopores are almost entirely absent. 



Figure 130 represents thin sections of a specimen with more abun- 

 dant mesopores, in fact, this specimen showed the hexagonal areas 

 of Dybowski's variety Jiexaporites. Aside from the mesopores, the 

 structure is exactly the same as shown in the preceding figure. 



Specimens from the Wesenberg limestone differ only in having 

 fewer diaphragms as shown in figure 131. In figure 132, Ulrich's 



