IHE BEY020A — BASSLER. 



361 



the internal structure can be determined -without the preparation of 

 actual thin sections, for by smoothing the surface of the bryozoan 

 and etching it slightly with acid most of the characters seen in 

 tangential sections become visible. A similar procedure for the 

 vertical section exhibits the internal characters very well. The aspect 

 of the surface and the structure seen in thin sections of a few species 

 of Trepostomata are illustrated in text figure 7. An outline of the 



17 18 



Fig. 7. — Structure of the Trepostomata. 



21 



1-4. Views of Dekayella praenuntia Ulrieh, a common Middle Ordovician bryozoan, show- 

 ing (1) the solid stony branch, X two-thirds; (2) surface of the same, XG, with the 

 spinelike aeanthopores and thin zooecial walls; (3) a tangential section, X12, illus- 

 trating- the same features in thin sections, and (4) a vertical section, >;C, with few 

 diaphragms in the immature zone to the right and numerous ones in the mature zone. 



5. Longitudinal thin section Xl2, of an Ordovician species (Hulloporina crcnulata 

 Ulrieh) with crenulated zooecial tubes in which diaphragms are absent in both the 

 mature and immature zones. 



U-8. Views of an Ordovician species (Ilallopora pulchclla Ulrieh) illustrating (6) a 

 fragment, X two-thirds, with tubercles developed at regular intervals on the surface; 

 (7) the surface, X 12, showing the zooecia and the intervening mesopores and (8) 

 vertical section, XI 2, with numerous tabulae in the mesopores. 



