G. W. Lee — Trepostomata. 
141 
■Carboniferous species with monticules have not yet been detected in Britain, but 
maculae are of frequent occurrence. These are extremely sporadic, and do not seem to 
obey any definite laws as regards their distribution within any given zoarium. These 
assemblages of modified zooecia, presumably for protective purposes, fall within the 
category of topomorphs 1 , a term recently suggested by Mr. W. D. Lang. 
Note on the interpretation of certain sections. —The sections must be made as thin 
as possible, as apart from the mere difficulty of observation, thick slides are apt to be 
misleading, and this is specially the case with longitudinal sections prepared with the 
view of observing the nature and distribution of the tabula?. 
Tabulae may be complete or perforated, that is, provided with a perforation which 
may be central, sub-central, or lateral. In longitudinal sections complete tabulae will 
appear as uninterrupted lines stretching from wall to wall. If in a well-made slide the 
plane of section passes through the lumen of a perforated tabula, the tabula will appear 
as two opposed lines separated by a central or sub-central interval, according to the 
position of the lumen. But if the thickness of the slide exceeds the diameter of the 
lumen the chances are that the appearance exhibited by a perforated tabula will be that 
of a complete tabula. This can be tested by the simple process of frequently examining 
an unfinished slide of a specimen known to belong to a species with perforated tabulae : 
it will be seen that while the section is still comparatively thick, the tabulae appear as 
■complete lines, gradually evincing signs of the perforation as the thickness diminishes. 
There is yet another case of misleading appearance which does not depend on the 
thickness of the slide, but on the position of the plane of section, which may often 
happen to pass outside the radius of the lumen, so that the tabula appears as complete. 
But in any given zoarium the number of zooecia, and consequently of tabula;, is 
generally so great that the perforation—if any—is bound to be shown at various 
points. 
In a number of British species the perforation is lateral, and occupies a distal 
position relatively to the direction of growth of the zoarium, so that a section cut 
normally to the direction of growth is more liable to be misleading than one containing 
the longer axis of the zoarium, as shown by diagrams, plate xiv., figs. 1 and 2. Very 
thick longitudinal sections cutting the zocecia-walls tangentially sometimes show a 
series of parallel lines roughly simulating tabulae; but they are really zones of 
specially dense sclerenchyma passing through the wall-substance. 
1 17, p. 68. 
