106 Prof. H. A. Nicholson—On the Favositide. 
that whatever is shown by a polished vertical or horizontal slice of 
a Coral will necessarily be shown by corresponding thin sections. A 
thin section often shows more than a polished slab, but never less. 
Every one who has ever examined polished specimens of such 
common Devonian species of Alveolites, as A. suborbicularis, Lam., 
A. Battersbyi, EK. & H., or A. reticulata, EH. & H., knows that 
such, as a rule, exhibit the mural pores with the utmost clearness, 
This being the case, it follows, as a matter of course, that thin 
sections of such forms show the mural pores, to one who knows 
what to look for, with at least equal clearness. What is true of the 
above-mentioned forms is true of the Favositoid Corals generally, in 
thin sections of which mural pores can usually be detected without 
any difficulty. This assertion is not at all affected by the fact that 
in certain states of mineralization any form of the Favositide may fail 
to yield direct evidence of mural pores, when examined either in 
polished slabs or in thin sections. 
The phenomena by which we may recognize in thin sections of 
the Favositoid Corals the presence of mural pores are, of course, 
well known to paleontologists generally. For the benefit, however, 
of those who may be beginning the study of the fossil Corals by 
means of thin sections, I may briefly indicate the character of these 
phenomena. 
In the first place, it is to be remembered what “mural pores” are. 
In their typical form, mural pores are simply rounded or oval apertures, 
arranged in longitudinal series, which perforate the walls of adjacent 
corallites and place adjoining visceral chambers in direct communi- 
cation. In some cases, the ‘‘mural pore” is not completed; since 
the thin ‘‘ primordial wall’ which separates adjacent tubes is not 
actually perforated, but is continuous. In such cases, the cavities 
of adjacent tubes do not actually communicate, and the so-called 
‘‘mural pore” is simply caused by a deficiency, at corresponding 
ee of the thick layer of secondary sclerenchyma (‘‘stereoplas- 
a”) which ordinarily coats both sides of the “primordial wall.” 
fh other cases, again, as in the genus Roemeria and in the typical 
species of Pachypora, where the above-mentioned layer of secondary 
sclerenchyma is excessively thick, the ‘‘mural pores” assume the 
character of longer or shorter tubes which connect adjacent visceral 
chambers. Whatever may be the precise form assumed by the 
mural pores, they can usually be readily recognised in thin sections, 
whether these sections be taken at right angles to the tubes or in 
a direction corresponding with the long axes of the latter. The 
facility with which they can be detected depends partly upon the 
condition of preservation of the specimen examined and partly upon 
the size and arrangement of the mural pores themselves. With 
regard to the latter point, the large, often uniserial pores of forms 
like Alveolites and Pachypora are usually more conspicuous than the 
generally smaller, often biserial or triserial pores of Favosites and 
its immediate allies. 
Bearing the nature of ‘mural pores” in mind, it is easy to recog- 
nize the phenomena which they present in thin sections. 
