272 Dr. H. A. Nicholson and Mr. R. Etheridge, Jun., on 



ened intervals between the latter. In the former case the 

 visceral chamber (fig. 2, A) is seen to be greatly contracted, 

 and to be reduced to a comparatively small rounded or sub- 

 polygonal central tube, which is in turn surrounded by a thick- 

 ened ring of sclerenchyma, which usually shows distinct 

 traces of its being composed of successively-deposited con- 

 centric laminae. In the latter case there is still a ring of 

 sclerenchyma within the dark outer polygonal boundary ; but 

 this ring is of small thickness comparatively, and the central 

 tube is wide and open. Further, in both cases alike there are 

 two phenomena observable which we are at present unable to 

 account for. One of these consists in the fact that the ring 

 of sclerenchyma within the corallite is never in contact with 

 the outer polygonal wall for more than one half or two thirds 

 of its circumference, being separated from the latter through- 

 out the remaining part of the tube by a distinct and conspi- 

 cuous interspace, which is filled in the fossil with transparent 

 calcite. Not only is this partial interspace between the inner 

 ring and the outer wall apparently always present (fig. 2, A), 

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

 the corallites in any particular section. The other inexpli- 

 cable feature is, that the outer dark walls of the corallites 

 appear to be always in close contact, whereas an examination 

 of the exterior of the tubes shows them to be only in contact 

 along the planes where thickenings of the wall are developed, 

 while they are separated by distinct intervals in the spaces 

 between them. 



Moreover, in many parts of tangential sections the corallites 

 exftibit few features that would satisfactorily separate them 

 from similar sections of certain Monticuliporce — 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 occa- 

 sionally observable which it is extremely difficult to explain ; 

 and we must admit that there are various points as to the 

 anatomy of this curious genus which must remain obscure 

 until a large series of specimens can be microscopically inves- 

 tigated. 



Longitudinal sections of the corallites (fig. 2, C) show the 

 periodical annular thickenings of the tubes in a very instruc- 

 tive manner, and demonstrate that these are really thickenings 

 of the wall, projecting both externally and internally; so that 

 it is not correct to regard the corallites 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, owing to its successively traversing 



