BRYOZOA. 



315 



I have detected in the tubes of several Palaeozoic CYCLOSTOMATA, 

 but unless it is terminal (see pi. XXIV, fig. 2,) the closest 

 search may fail to reveal it. 



Fig. 5. Six illustrations showing various forms of opercular covers, x 50. a, perfect 

 aperture of Pinnatopora stellipora Y. & Y. b, perfect aperture of Callopora ramosa 

 d'Orb. c, aperture of Stenopora tuberculata Prout. d, aperture of Polypora bisermta, 

 having the central perforation of the cover closed, e, Meekopora clausa UL; /, Fistuli- 

 pora foordi UL 



In typical CYSTODICTYONID.E and some FISTULIPORIDJE (e. g. F. 

 foordi, PI. XLVIII, fig. 7,) the larger portion of the aperture is 

 often found closed by a plate. The remaining open portion is 

 of circular outline and occurs immediately within the lunarium. 

 In Meekopora, clausa (pi. LXXVII, fig. 7,) and other FISTULJ- 

 PORID.E, the perforation in the opercular plate is sub-central. 

 When in a good state of preservation the zooecial tubes of 

 Callopora, Stenopora and several species of Batostomella are 

 closed by a centrally perforated opercular structure. Ordinarily 

 the perforation is surrounded by a slight rim and the whole 

 plate may be either slightly concave or convex and smooth. In 

 Cnllopora elegantula Hall, C. ramosa d'Orb, and other species 

 of the genus, six to ten small ridges radiate from the perforated 

 centre to the margin, giving the false appearance of a septate 

 aperture. These structures, as they occur in the TREPOSTOMATA, 

 are most certainly not developed only at the final period of 

 growth, as some authors have thought, but we have abundant 

 evidence to show that what at one time served the purpose of 

 a cover to the zocecium, next formed the floor of the succeed- 

 ing cell; in other words, became a "diaphragm." Furthermore, 

 there appears to be no reason to doubt that this process was 

 carried on throughout the extent of the "mature region" since 



