AND GLYPHASXR.EA, DUNCAN (1887). 213 



compact, but consists of minute, dull, whitish grains of calcite, which 

 have a powdery, incoherent appearance. There is a well-marked 

 contrast between this and the compact structure of the outer layer 

 of the lamina ; and from a comparison with the structure of the inner 

 septal layer in other corals, it seems probable that the powdery, inco- 

 herent material in the fossils is not the original condition of this 

 layer, but results from subsequent change or disintegration during 

 fossilization. 



Exterior to the lattice-like layer, and in intimate union with it, 

 is the outer layer of the septal lamina, which is composed of com- 

 pact, minutely fibrous, calcareous tissue, known as stereoplasm. 

 This fibrous layer gives lively prismatic colours when viewed between 

 crossed nicols. The fibres are deposited in continuous, excessively 

 thin layers, which are indicated by slightly darker lines or breaks. 

 The microscopic appearance of the stereoplasm in sections of the 

 fossil Septastrcea varies but little from that of recent corals of the 

 genus Flabellum, for example. 



The outer or exposed surface of the septal lamina, formed by the 

 stereoplasm, is smooth, solid, and compact, and has the appearance of 

 pohshed ivory. Even in the thinnest septa the stereoplasmic layer 

 forms the largest part of the septal laminae ; and in the process of 

 growth, and more especially in the mature corallites, it is thickened * 

 by repeated depositions of the same material. The inner or lattice- 

 like layer of the laminae, on the other hand, does not receive any 

 subsequent additions of growth. 



In transverse sections it not unfrequently happens that the central 

 or inner layer of the septum is only represented by an empty space 

 enclosed between parallel layers of stereoplasm, thus showing the 

 less resistant nature of the substance of the central layer in com- 

 parison with the stereoplasm. 



Structure of the Theca or CoralUte-wall. — It has been pointed out 

 that the wall of the corallite is merely the outer extension of the 

 laminae of the septa ; and its minute structure, in the main, resembles 

 that of the laminae. The exterior or lateral surface of the corallite- 

 wall in S. Forhesi, as seen in fractured specimens, exhibits a series 

 of parallel, vertical, slightly depressed lines or furrows, with slightly 

 rounded or nearly plane longitudinal interspaces, or pseudo-costae t. 

 Examined by a strong lens, these interspaces are seen to consist of 

 closely arranged, directly transvei^se or slightly arched ridges or 



* Prof. Duncan suggests that the axial or parent corallites, as he styles them, 

 have undergone some diminiition in the bulk of their walls and septa, probably 

 during the life of the polyps {I. c. p. 25); but I can see no evidence that the 

 stereoplasmic layers have been removed after having been once formed, or that 

 the wall and septa of the lower portion of the corallites have been originally 

 thicker than they are now. 



+ The term "costae" has been applied to the edges or margins of the septa 

 which project on the exterior of the thecd or corallite -wall, and they are merely 

 the distal edges of the septa. But septa, formed on the type of Septasfrcea and 

 Flabelhtm, do not project beyond the so-called theca, and the spaces, where they 

 appear on the exterior, are depressed linear furrows instead of projecting 

 ridges or truecostae. The flattened or rounded interspaces between the septa of 

 these corals, which stand out slightly in relief, are generally termed pseudo-costce. 



