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 Dasniia 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, papillae, spines, and granules, which are variously 

 arranged in radiating, parallel, or irregular series. 



The structure of the laminae differs in many species. The laminae 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 (he septa are marked more or less by the dissrpimcnts^ and tahuloe^ 

 and they give origin to these structures as well as to the synapticidcB? 



Note. — The description of the septa of the Rugosa is omitted until the introduction to the palseozoic 

 corals is commenced. For an 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, maybe found in 

 his ' Introduction a I'etude des Polypiers Fossiles," p. 18. 



1 Plate I, figs. 12, 13, 15 ; Plate IV, fig. 4. ^ pi^te III, fig. 16 ; Plate IV. fig. 2. s pj^te III, fig. 2. 



