BULLETIN 7*7, UNITED STATES NATIONAL MUSEUM. 



rather large, subpolygonal to rounded according to the number of 

 mesopores, thm-walled, five in 2 mm. Mesopores exceptionally small 

 and numerous, the interzooscial spaces sometimes being filled by 

 three distinct rows of them, Acanthopores wanting. 



Vertical sections show erect tubes, in general perpendicular to the 

 basal epitheca, with a very short, inconspicuous immature region. 

 These zooecia are crossed by numerous, more or less curved dia- 



FlG. 109.— MESOTKYPA MILLEPORACEA. a, the EPITHECAL side of a fragmentary ZOARroM, NATURAL 

 size; & AND C, A TANGENTIAL SECTION, X20, AND SEVERAL ZOCECIA, X35, THROUGH THE REGION IN WHICH 

 THE MESOPORES ARE SMALLEST AND MOST NUMEROUS; d, TANGENTIAL SECTION OF THE SAME SPECIMEN, 

 X20, SHOWING LARGER, LESS NUMEROUS MESOPORES; e, VERTICAL SECTION, X20, PASSING THROUGH 

 PORTIONS OF TWO LAYERS OF ZOCECIA. ChASMOPS LIMESTONE, SOUTH OF BODAHAMN, ISLAND OF OELAND. 



phragms with three to five in a tube diameter. A variety of appear- 

 ances is presented by these diaphragms in such thin sections. Some 

 are straight, others are slightly obhque, while still others are curved 

 as much as in the ordinary cystiphragm. The mesopores are always" 

 numerous, but vary in number and size at different stages of growth. 

 In the early portion of the mature region they are seen as single 

 tubes crossed by thin diaphragms at intervals varying from one to 

 two times their own diameter. Their appearance and number at 

 this stage, as shown in tangential sections, is indicated in figure 109 d. 

 In the later stages of the mature region their diameter decreases and 



