316 PALEONTOLOGY OF ILLINOIS. 



in tangential sections of these forms the opercula of previous 

 cell-layers are often recognizable. Their recognition is especially 

 easy in Stenopora, since in species of that genus the central per- 

 foration generally remains open. Occasionally an exceedingly 

 thin membrane is stretched across. In Callopora the perfora- 

 tion is closed by a secondary deposit and cannot be detected 

 excepting when it differs in color from the rest of the plate. 

 The calcareous plates which cross the tubes in the immature 

 region of the zoarium in the Bryozoa under consideration, seem 

 to have been without perforations of any kind, but so far as we 

 can determine, they are precisely like the true "diaphragms." 

 The latter are probably the most characteristic feature of the 

 TREPOSTOMATA, occurring, perhaps without any exception, in 

 all the members of this suborder. They consist of simple calca- 

 reous plates, flat, or convex, which cross the zocpcial tube at 

 near a right angle. In all cases they occur at shorter intervals 

 in the mature region than in the immature, while in the meso- 

 pores the diaphragms are also comparatively more numerous 

 than in the zocecia. Idiotrypa parasitica is an exception to the 

 last rule, since in that species the diaphragms occur upon the 

 same level in all the tubes. In Dekayia and Dekayella, at 

 certain periods in the growth of the zoarium, a thin pellicle is 

 drawn over greater or smaller patches of the surface, while 

 other portions have the cell-apertures open. This membrana- 

 ceous covering was doubtlessly developed at the final period in 

 the existence of the zooids of each layer of cells, and after form- 

 ing the floor of the succeeding layer, ultimately became a 

 diaphragm. 



True diaphragms are of very rare occurrence among the 

 Cryptostomata, but in species of Ptilodictya Lonsdale (Hetero- 

 dictya Nich.), the larger specimens are composed of more than 

 one layer of cells. Each layer is marked by an inferior and 

 superior hemiseptum (just the same as the original cell) which 

 in vertical sections might be mistaken for diaphragms. 



