214 DR. G. J. HIXDE OX SEPTASTEJEA, d'oRBIGXY (1849), 



growth-lines, which dip deeply downwards at each vertical furrow, 

 and, apparently, only extend from furrow to furrow (PI. IX. fig. 12). 

 At the depressed lines or furrows these transverse growth-lines of 

 the exterior of the theca are continuous with the growth-lines which, 

 as already mentioned, form the inner or median plane of the septal 

 lamina. The vertical furrows, therefore, are coincident with the 

 median line of the septa, and the thecal surfaces between the furrows 

 or pseudo-costae are between the septa. The transverse growth-lines 

 of the outer wall of the theca are, however, much stronger than the 

 growth-lines of tbe septal laminae with which they correspond. 



Within the growth-lines of the thecal wall there is a lattice-like 

 tissue of the sauie nature, but more delicate than that of the septal 

 lamina ; and, united to this, is a layer of stereoplasm, which is con- 

 tinuous with that forming the outer layer of the septa. 



The constitution of the thecal wall is therefore similar to that of 

 the septal laminae ; there are first, from without inwards, the trans- 

 verse growth-lines which, in connexion with the vertical lines of 

 the lattice-like tissue, form the primary layer of what is now a 

 powdery, incoherent material, and next, the layer of stereoplasm. 

 In the wall of the theca the primary layer is exterior to the coral- 

 lite, whereas in the septa the primary layer of each lamina is in the 

 central or inner portion. 



The stereoplasmic layer thus forms a complete interior or lining 

 to the cor alii tes. In the young corallites, as weU as in the full-sized 

 forms which have not attained their full growth-limits, the stereo- 

 plasm forms a simple thin covering to the dehcate interior membrane ; 

 but when the full growth-hmit of the coraUites is reached, this 

 material is deposited in successive layers within the interlocular 

 chambers above the uppermost dissepiment until they are filled nearly 

 to the surface of the corallum (PI. IX. figs. 13, 14). The deposit 

 is, in all cases, laid down from within ; the close apposition of the 

 corallites prevents any addition to the exterior of the walls. 



The horizontal dissepiments which extend across the interseptal 

 loculi and successively form the basal floor to the living polyp 

 appear to consist primarily of a single, delicate, calcareous membrane, 

 of a fibrous structure, which grows from the margins of the loculus 

 towards the centre (PI. IX. fig. 15). Whether this primary layer 

 is of the nature of the primary layer of the septa and theca, or 

 whether it is of stereoplasm, I have not been able to determine ; 

 in either case it becomes subsequently thickened by stereoplasm in 

 common with the other skeletal tissues. 



The nature of the septa and theca in recent corals and their re- 

 lation to the tissues of the living polyp have lately been carefully 

 studied by several German and English authorities, and the facts 

 shown by the fossil Septastrcea have an important bearing on the 

 subject. In the study of the minute structure of recent corals, 

 transverse microscopic sections have been almost exclusively relied 

 on. The septa of such forms as Flabellum, Lophohelia, and Mussa 

 show, when viewed by transmitted light, a central dark line or layer, 

 of a confused appearance, which is surrounded on both sides by a 



