230 



BULLETIN 11, UNITED STATES NATIONAL, MUSEUM. 



zooecia and more numerous mesopores present at regular intervals 

 but not conspicuous. The shape of the zoarium is well shown by 

 Eichwald's figure here reproduced. 



Zooecia and mesopores often similar in shape and size, each with 

 thin walls and polygonal apertures. The zooecial apertures, however, 

 are usually more regularly polygonal and sUghtly larger than the 

 mesopores which frequently are numerous enough to completely 

 isolate neighboring zooecia. Another distinguishing character between 

 the two at the surface is the frequent occurrence of covers on the 

 mesopores, the last diaphragm in this case serving as a closure. True 



Fig. ]27. — DiANULITES FASTIGIATUS. a, SIDE VIEW OF A ZOAEIUM, SHOWING THE EPITHECA; 6, TOP OB 

 CELLtTLITEROUS SXJEFACE, ENLARGED; C, EDGE VIEW OF A BROKEN" FRAGMENT, ENLARGED, ILLXISTRATING 

 THE UNEVEN FRACTURE; d, THE T"5TE-SPECIMEN OF DiANULITES DETRITUS ElCHWALD, NATURAL SIZE. 



"Calcaiee 1 Orthoceratites," Reval (fig. a), and Pulkowa (fig. d), Baitic RussLi. (After 



ElCHWALD.) 



acanthopores absent, but the granular structure characteristic of the 

 genus is well marked. 



The similarity of the zooecia and mesopores is equally great in thin 

 sections. In vertical sections the wall structure of the two is the 

 same and their diameter is frequently equal. There need be no 

 difficulty in distinguishing the two, however, since diaphragms are 

 practically absent in the zooecia but occur always in the mesopores at 

 intervals varying from two to five times their own diameter. 



The differences between the zooecia and mesopores at the surface, 

 mentioned above, hold in tangential sections, but here another means 

 of distinguishing them is presented. The zooecial cavities being 

 practically mthout diaphragms, are very frequently filled with clay, 

 the dark color of which, in sections, is in contrast with the clear calcite 

 filling the diaphragm-bearing mesopores. 



