70 Cunnings — Development of some Paleozoic Bryozoa. 



It is not unlikely that the form and position of the earinee and 

 carinal superstructures (scalse, etc.) is very largely determined 

 by the same necessity which, in the writer's opinion, has, in 

 the case of the Brachiopoda and Pelecypoda, given rise to 

 shell plication, namely, that of admitting water and exclud- 

 ing enemies. The protection afforded by such structures as 

 the scalse of TJnitrypa and the still more elaborate super- 

 structure of Hemitryjpa? is very complete. Of course any such 



# 



/?•--' 



Figure 70. — Base of a neanastic Unitrypa, showing obliquity of trans- 

 verse plates (scalae), and thickened and modified representative of initial cup. 

 x20. 



advantage is gained at the corresponding disadvantage of less 

 perfect circulation of water over the zooecid than is the case 

 where they are unprotected. 



IV. Development of Polypora. 



Nepiastic Stages. 



Ananepiastic Stage. — The earliest observed stage of Poly- 

 pora consists of the protcecium and five encircling zocecia (figs. 

 71-77). Even at this early stage the primary branches, seven 



* Thin sections indicate that the early nepiastic stages of Hemitrypa are 

 identical with those of Fenestella. 



