248 PROFESSOR NICHOLSON ON THE MODE OF GROWTH AND INCREASE 
inversely conical mass, the broad upper end of which is convex ; and it is only 
at this upper end that the calices are actually open, or the corallites are truly 
alive. The whole of the lateral surfaces of the colony exhibits calices also, 
but these are covered by a well developed epitheca, through which they can 
be more or less dimly discerned. 
Another circumstance which has a good deal to do with the growth 
and increase of the compound coralla, is the presence or absence of a 
coenenchyma, and the extent to which it is developed when present. In 
none of the compound Paleozoic corals is the ccenenchyma a very con- 
spicuous feature, and it is usually altogether wanting. Even when present, 
there may reasonably be entertained considerable doubts as to its true 
nature. In Heliolites and its Paleozoic allies it is possible that we have 
to deal with a true coenenchyma, comparable to that of Millepora. In 
these cases the corallites are very markedly larger than the tubuli of the 
coenenchyma, and are further distinguished by being septate. In Fistulipora, 
again, we have a transitional form, the corallites not being very greatly larger 
than the so-called coenenchymal tubuli, and agreeing with them precisely in 
structure. The question arises, therefore, whether these tubuli are to be 
regarded as constituting a proper coenenchyma, or whether they are not really 
of the nature of aborted or rudimentary corallites. Several facts would favour 
the latter view. Thus, in Constellaria (Stellipora), and in some species of 
Callopora, the difference between the larger and smaller tubes is not very 
striking, and the latter may well be nothing more than imperfect corallites. 
In many species of Chetetes (Monticulipora), further, the corallum is composed 
of three sizes of tubes, some unusually large, others of medium dimensions, 
and others very minute. The large tubes are comparatively few in number, 
are usually aggregated into definite groups, and are undoubtedly corallites. 
The medium-sized tubes form the bulk of the corallum, and are also undoubtedly 
corallites. The smallest tubes may be placed simply at the angles of the 
normal calices, when they would seem to be certainly young or rudimentary 
corallites ; or they may be collected as well into groups occupying definite 
spaces. In this latter case it is uncertain whether they are to be regarded as 
being small corallites, or as belonging to the coenenchyma, though I strongly 
incline to the former view. Additional support is given to this view by a 
consideration of species like Favosites hemispherica, Yand. and Shumard, and 
F. dubia, De Blainville. In the first of these we find, in many specimens, a 
most marked and extraordinary singularity in the size of the calices, but not 
at all as is seen in such a species as /. Forbesi, where the small calices are 
simply scattered irregularly amongst the larger ones. On the contrary, we 
have here the curious phenomenon that whilst the majority of the corallites 
have a certain average size, there are whole tracts, quite irregular in shape and 


