BRYOZOA. 



309 



thin sections of such forms, of which Monotrypella pulchella Ed. 

 & H.. Amplexopora cingulata, Anisotrypa symmetrica, Steno- 

 pora carbonaria "Worthen, Ptilodictya pavonia D'Orb., Nema- 

 topora lineata Billings, Pachydictya everetti, Batostoma varia- 

 bile and many others are examples, the visceral chambers of 

 each zooecium is seen to be surrounded by its own calcareous 

 investment, and to be separated from its immediate neighbors 

 by a more or less clearly marked dark or light line. The 

 actual thickness of the walls inside of the divisional line varies 

 with the species, but in a comparatively small number of forms, 

 represented by the species like Monotrypa rectimuralis, Diplo- 

 trypa petropolitana Pander, and D. patella, they are so thin 

 that their duplex character cannot be made out in thin sections, 

 appearing in them merely as delicate dark lines. However, as 

 Nicholson truly says, "rough fractures will demonstrate what 

 thin sections fail to show.'' In rough fractures of the zoaria of 

 these species the tubes always separate clearly from one another, 

 each carrying with it its own wall, a phenomenon which could 

 not be possible if the primitively duplex condition of the walls 

 of adjoining tubes had in reality been destroyed. 



In another group of forms comprising mainly thick walled 

 species, and, with others, the species of Batostomella, Leioclema, 

 Atactopora, many of Rhombopora and several of Stenopora, 

 there is no distinct divisional line in the centre of the partition 

 between contiguous tubes and the walls appear to be actually 



Fig. 4. Vertical sections x50, showing variations in wall structure, a, Monotrypella 

 cequalis UL; 6, Stenopora americana UL: c, Amplexopora cingulata Ul.; d, Monotrypa 

 rectimuralis UL 



