146 
G. W. Lee — Trepostomata. 
certain shapes being typical of certain genera. For instance, a beaded appearance due to 
periodical cessations of thickening are typically developed in species of Stenopora and 
Tabulipora and was long, thought to be restricted to the former, but this feature is now 
known to occur in .certain Lower Palaeozoic forms, e.g., Stigmatella spiriosa IT. & B. and 
Dekayia subfroudosa Cumings. 
- Tabulae— -The tabulae offer in the British forms very reliable characters. Their 
distribution hardly varies within each species, while their nature, that is, whether they are 
perforate or complete, seems to be absolutely constant. No British species has yet been met 
with in which the tabulae are not all of the one type. Strictly speaking there would be 
an exception in the case of Tabulipora , in which a concave plate sometimes covers the 
perforation of the tabulae. It is possible that this curious body was always present, 
though not actually attached to the tabulae, and preserved only in exceptional cases. 
Some of the old diagnoses imply the irregular occurrence of both complete and perforate 
tabulae in the same zooecium, but this is doubtless to be attributed to faulty observation, 
excepting of course those forms in which the two types of tabulae are distributed accord¬ 
ing to a definite arrangement, as in Hemiphragma. 
Acanthopores. — Considerable taxonomic value is attributed to the acanthopores. 
Some caution must be exercised when examining tangential sections, since the appearance 
presented by the acanthopores necessarily varies in some degree according to whether the 
plane of section cuts them near their point of origin or near their distal end. Once due 
allowance is made for this, it will be seen that they offer fairly reliable characters, varying 
within each zoarium not so much in their size as in their distribution. 
Surface features. —The surface features of the zooecia, as exhibited by the British 
Carboniferous species, are generally too variable and equivocal to be of service. For 
instance the thickness of the partition between the zocecial apertures varies, in the same 
branch, within wide limits, and the same applies to the spine-like projections of the 
acanthopores. Species based on these surface-characters alone are unrecognizable, as for 
example Heterotrypa delicatula Vine, which was based on features common to widely 
different species. 
Mode of Growth.— As already stated, the ratio of axial region to diameter, in 
cylindrical forms, is an excellent criterion. In the majority of the species the peripheral 
portion of the zooecia is at right angles to the surface. There are, however, some species 
in which that portion is bent at a wider angle, and since no forms have yet been observed 
showing the co-existence of the two modes, specific importance is here given to the angle 
at which the zooecia bend in order to reach the surface. 
