GENERAL AND COMPARATIVE STRUCTURE. 53 



Whether the tabulae assume the form just described, or are 

 of the more normal type, it is usual to find in the dimorphic 

 coralla that there is a marked difference in the tabulation of 

 the large and small coral! ites respectively. It is usual, namely, 

 to find that the tabulae of the small corallites (which are always 

 " complete " and approximately horizontal) are conspicuously 

 more numerous and more closely set than is the case in the large 

 tubes. In other cases, again, even where clusters of large coral- 

 lites constitute a noticeable feature (as in AI. pulchclla, E. and 

 H.), there is no recognisable difference in the tabulation of any 

 one series of corallites as compared with any other. More- 

 over, there is evidence, in some cases (as, for example, in M. 

 tindulata, Nich.), of a distinct periodicity in the production of 

 the tabulae, these structures being comparatively few in number, 

 and being mostly placed at corresponding levels in contiguous 

 tubes, thus more or less perfectly dividing the corallum into a 

 series of concentric layers. Lastly, in those species of the 

 group which have the corallum conspicuously divided into an 

 axial and a peripheral zone, it is the rule to find the tabulae 

 very sparingly developed, or even wanting, in the central and 

 deeper portions of the corallum, and becoming numerous as 

 the corallites approach the surface. 



VII. Septa and Pseudo-septa. — Nothing of the nature of 

 " septa," in the proper sense of this term, has ever been de- 

 tected in any Monticuliporoid ; and we do not even meet with 

 any structures that could be compared with the imperfect 

 spiniform septa of so many of the Favositidce. In a few forms 

 (such as J\L iiiiplicata, Ulrich), the tubes seem to be indented on 

 one or more sides by blunt inward projections, which might, on 

 cursory inspection, be regarded as of the nature of septa ; but 

 thin sections show that in these cases the apparent septa are 

 caused by inward protrusions of the walls of the tubes, due to 

 the development of "spiniform corallites." In one or two 

 other cases {e.g., III. pavonia, D'Orb.), I have occasionally 

 detected a blunt, tooth-like projection into the interior of the 



