10 BRITISH FOSSIL CORALS. 
There is every possible variety in the size of the septa; but, as has already been 
mentioned, the primary are the largest, and the members of the higher orders the 
smallest. 
The same observation holds good, as a general rule, with regard to the height. The 
exsertness of septa varies greatly ; some are arched and extend far higher than the top of 
the wall, and others do not extend upwards above the wall at all. The longer septa in 
some species meet and are twisted centrally, whilst those of the higher orders only just 
project within the calice. 
The breadth of the septa depends very much on the habit and size of the corallum; 
the bi-laminate arrangement is very distinct in some species, whilst in others it cannot be 
seen, and the septa are thin, delicate, and very fragile. The genus Dasmia has a tri- 
plated arrangement of the septa. 
The thickness of the septa varies in corals of the same genus, and it becomes of some 
importance in a diagnostic sense. 
Usually all the septa are thickest at their origin from the wall, they then thin off 
towards their inner edge, but very often there is an increase of their bulk near the 
columella and midway. 
The ornamentation consists of ridges, papillze, spines, and granules, which are variously 
arranged in radiating, parallel, or irregular series. 
The structure of the laminze differs in many species. The laminee may be dense and 
imperforate, or more or less perforate generally or only in certain parts. In some corals the 
septa are mere spiny processes, in others they are spongy in appearance, and in the other 
extreme they are very dense and solid. 
The upper or superior margin of the septa is free, and the inner margin or end is 
towards the columella or long axis of the corallum. The upper margin may be smooth 
or incised, lobed or entire, granular or largely dentate, serrate and spined; it may 
be arched, or may be directed downwards and inwards, and it may be enlarged at any 
part. 
The inner end or margin may be free, may join a columella by processes of dense or 
of lax hard tissue,—may send off processes to form a columella, with others from other 
septa,—may be attached to pali,—and it is often very ragged, twisted, clubbed, and 
perforate. 
The inner ends of small septa may become attached to the sides of the larger. 
Finally, the sides of the septa are marked more or less by the dissepiments' and tabula,’ 
and they give origin to these structures as well as to the synapticule.® 
Nore.—The description of the septa of the Rugosa is omitted until the introduction to the palzeozoic 
corals is commenced. Foran exhaustive essay on the septa, see Milne-Edwards and J. Haime, ‘ Hist. Nat. des 
Corall.,’ vol. i, p. 40. M. E. de Fromentel’s criticisms on it, and his own able descriptions, may be found in 
his ‘ Introduction a |’étude des Polypiers Fossiles,’ p. 18. 
1 Plate I, figs. 12,13,15; PlateIV, fig.4. % Plate III, fig. 16; PlateIV, fig. 2, % Plate III, fig. 2. 
