GENERA OF FAVOSITIDyE. 175 



wall for more than one half or two-thirds of its circumference, 

 being separated from the latter throughout the remaining part 

 of the tube by a distinct and conspicuous interspace, which is 

 filled in the fossil with transparent calcite. Not only is this 

 partial interspace between the inner ring and the outer wall 

 (PI. IX., fig. I a, and fig. 26, a) apparently always present, but 

 it seems to be always situated upon the same side of all the 

 corallites in any particular section. The other point that is 

 difficult to understand is how the outer dark walls of the coral- 

 lites should appear in tangential sections to be always in close 

 contact, seeing that an examination of the exterior of the tubes 

 with a lens shows that they are only in contact along the thick- 

 ened planes in which rings are developed, and are separated by 

 distinct intervals in the spaces between these. Moreover, in 

 many parts of tangential sections the corallites exhibit few 

 features that would satisfactorily separate them from similar 

 sections of the tubes of certain Montiatliporcs, though they 

 usually have exceptionally thick walls, and also often exhibit a 

 thin dark ring a little within the true wall, and concentric with 

 the latter. There are also some other phenomena occasionally 

 observable which it is extremely difficult to explain ; and it 

 seems clear that the precise structure of this curious type must 

 remain to some extent unelucidated until a large series of speci- 

 mens can be microscopically investigated. 



Longitudinal sections of the tubes (fig. 26, c, and PI. VIII., 

 fig. i) show the periodical annular thickenings of the tubes 

 in a very instructive manner, and show that these are really 

 thickenings of the wall, projecting both externally and internally, 

 and that it is therefore incorrect to regard the tubes as being 

 " periodically constricted " — this phrase applying only to the 

 visceral chamber. In fact, the longitudinal section of the wall 

 has a regularly moniliform appearance, due to its successively 

 traversing thickened and unthickened segments of the tube. 

 Sections of this kind also show that there exist remote and 

 complete tabula:, which are usually placed at approximately 

 corresponding levels in all the corallites of a single colony. 



