216 DE. G. J. Him)E ON SEPTASTK^A, d'oBBIGNT (1849), 



likewise point out, in many existing corals, the dark line in the 

 centre of each septum as the centre from which calcification has 

 taken place, therefore the primary layer. 



The structure of the septa in the fossil Septastrcea fully confirms 

 the view of Lindstrom that the central or dark layer of the septum 

 is distinct from the enclosing or lighter layers ; but the fossil coral 

 proves the further fact, which does not appear to have been pre- 

 viously ascertained either in fossil or recent corals, that the delicate 

 central dark layer in the septum is really divided by a median plane, 

 and consists of two distinct halves or laminae, and that the structure of 

 each of these,' with its outer envelope of stereoplasm, is similar in 

 this genus to that of the theca, which, indeed, in this and some other 

 genera is merely a lateral extension of the septal laminae. 



This median division of the primary layer of the septum likewise 

 accords with the view, enunciated more particularly by von Koch, 

 that the septa, in common with the other hard skeletal tissues, are 

 formed by a secretion of the ectodermal layer of the living polyp, 

 and that, morphologically, the skeleton is exterior to the soft tissues 

 of the animal. To within a recent period the statement of Edwards 

 and Haime * that the septa and theca were produced by the meso- 

 dermal layer of the polyp was generally accepted ; but v. Koch has 

 conclusively shown that the septa are developed within longitudinal 

 folds of the ectoderm by modified cells, technically known as '' cha- 

 licoblasts " f. 



The discovery of the median division of the primary layer in the 

 septa of Septastrea led to an examination of other genera of fossil 

 corals, and I find a precisely similar structure in fossil species of 

 Flabellum, Ceratotroclius, Trochosmilia, and Paracyathus. There is 

 but little doubt that the same structure is present also in Conosmilia, 

 Duncan ; but I have been unable to examine examples of this coralj. 



In many respects there is a close correspondence in the structure 

 of the septa and theca of Septastrcea and that of the recent and fossil 

 genus Flabellum. The septal laminae of this latter genus are con- 

 tinued laterally outwards to form the theca ; the transverse growth- 

 lines of the primary layer of each lamina are similar in both genera ; 

 and there is a precisely similar deposition of stereoplasm, which 

 forms a solid infilling in the basal portion of Flabellum, as in the 

 upper portion of the corallites of Septastrcea. These resemblances 

 in the fundamental structures of the septa and theca appear to me 

 to indicate a closer relationship than mere similarity in the number 

 or disposition of the septa or other superficial characters ; and there 

 is some reason for believing that a thorough investigation of the 

 minute microscopic structure of fossil and recent corals will aff'ord a 

 more satisfactory basis for their classification than that now in 



* Hist. nat. des Coralliaires, t. i. p. 34. 



t See von Heider, ' Sitzungsber. kaiserl. Akad, d. Wiss. Wien,' 1882, p. 651. 



I This genus was described in tlie Ann. & Mag. Nat. Hist. 1865, vol. xvi. 

 p. 184, and three species included in it. The type specimens were stated by 

 Prof. Duncan to be in the Collection of the Geol. Soc, They were redescribed 

 and refigured in the Quart. Journ. Greol. Soc. vol. xxvi. 1870, p. 305. by the same 

 author ; but the oi'iginals can no longer be found in the Geol. Society's Museum. 



