212 BR. Gr. J. HIIS^DE ON SEPTASTR^A, d'oEBIGNY (1849), 



of ordinary writing-paper, will be seen to consist of two laminae, which 

 are so closely apposed that to ordinary sight they appear as a simple 

 thin plate. Closer examination shows that between these laminae 

 there is a delicate central linear interspace. The bilaminate septum 

 can be traced to the periphery or bounding wall of the coraUite, and 

 it does not then terminate, for one of the laminae bends round 

 sharply and is continuous with, and forms the theca or corallite- 

 wall on the one side, and the other lamina of the septum similarly 

 forms the thecal wall on the opposite side*. On tracing these 

 lateral extensions further on both sides, we find the same process 

 repeated at the next septum, for there is a similar sharp inflexion 

 to form the septal laminae, and the same structure is repeated aU 

 round the corallite (PI. IX. figs. 7, 8, 16). 



The bilaminate nature of the septa is stiU more clearly shown in 

 longitudinal fractures which occur in broken extremities of the 

 stems and branches of difi'erent specimens. In these cases the septa, 

 though only of paper thickness, individually split up longitudinally 

 into distinct symmetrical halves, and the median plane passes 

 through the centre of that delicate interior structure already noticed, 

 so that each of the two laminae of the septum consists of two distinct 

 layers of material, an outer and an inner ; the two inner layers 

 being very intimately united, so that they have hitherto been re- 

 garded as forming a single distinct central layer in the septum. 

 That this median septal structure consists of two distinct halves is 

 further shown from the fact that in several cases corallites actually 

 split along the median plane of the septa, and not in the interspaces 

 between them (PI. IX. fig. 9). 



The inner or median face of each of the laminae of the individual 

 septum is a nearly plane calcareous membrane, exhibiting somewhat 

 broad, curved, transverse growth-lines, which are highest at the 

 peripheral margin of the septum, next the outer wall, then curve 

 downwards and again rise slightly to the axial or inner margin 

 (PI. IX. fig. 9). These transverse growth-lines have much the 

 same general appearance as those which usually occur on the outer 

 surface of the theca in most ordinary corals, and are generally 

 known as epitheca. In immediate connexion with the transverse 

 growth-lines there is a delicate layer, consisting of well-marked 

 linear ridges, slightly fiexuous, and parallel with each other, which 

 are either vertical or oblique in direction, so as to be at right angles 

 to the transverse growth-lines (PL IX. fig. 10). Between the 

 vertical ridges there are very minute linear furrows, which in the 

 best-preserved specimens are crossed transversely, so as to produce 

 a linear series of minute holes, thus giving this layer a lattice-like 

 appearance (fig. 11). On the split face of certain septa, the surface 

 layer of transverse growth-lines is not preserved, and only the ver- 

 tical linear ridges are shown on each half of the septum. 



The ridged or lattice-like inner layer of the septal lamina is not 



* This extension appears to be uninterrupted, and I have failed to discoyer 

 any sutural hne between the septal laminae and the thecal extension. 



