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PALEONTOLOGY OF ILLINOIS. 



mature. I have seen specimens of these species in which it was 

 possible to distinguish more than twenty successive zones, with 

 out a break in the continuity of the tubes. A separation of 

 the zoarium into different zones is apparently not possible with 

 the true species of Monotrypa (e. g. M. undulata Nich., M. rec- 

 timuralis). In these extremely simple forms the walls of the 

 tubes remain excessively thin throughout their length, the dia- 

 phragms are separated by wide and unequal intervals, and 

 neither mesopores nor acanthopores are present. 



b. Independence of the Zocecia walls. Theoretically it may 

 be said that the zocecia of all the Palaeozoic Brj'ozoa had per- 

 fectly independent and complete walls. In many cases this in- 

 dependence of the wall of each individual cell or tube is clearly 

 preserved throughout the entire growth of the zoarium and in 



Fig. 3. Tangential sections of Palaeozoic Bryozoa showing variations in wall struc- 

 ture, x50. a. Monotrypa rectimuralis Ul., showing the extreme simplicity that marks 

 that genus. 6. Monotrypella cequalis Ul., showing structure of walls just beneath the sur- 

 face, c, a similar section of Amplexopora cingulata UL, showing slightly more complex 

 structure, d, similar section of Callopora subnodosa UL, showing apparent amalgama- 

 tion of walls, e, Arthropora shafferi Meek, /, Heliotrypa bifolia UL, showing the most 

 complicated structure yet seen. 



