42 SIDERASTREA RADIANS. 



the center of its own calice (plate 10, fig. 63). The thecal ridge is thus the 

 area of divergence of the striae of adjacent septa. It is further found that 

 the striae on opposite sides of the same septum correspond with one another, 

 as can be seen by looking edgewise at the inner margin of an individual 

 septum ; hence opposite striae terminate in one serra. The synapticula and 

 granules are set mainly along the ridges of the septal face, but the former 

 may be so broad as to extend over more than one stria. 



In many corals lighter lines run transversely across the striae, parallel 

 to the dentate septal edge, and mark successive septal margins, corresponding 

 to distinct periods of growth in the life of the polyp. The part between two 

 successive curves represents the addition made to the septum during a single 

 period of polypal growth, and is called by Miss Ogilvie (p. no) "the growth- 

 segment of the septum " or " a septal segment." The septa of Siderastrea 

 very rarely exhibit such lighter cross lines ; by Miss Ogilvie the synapticula 

 are considered an essential part of every growth segment in this genus. 



A magnified transverse section through a portion of the corallum shows 

 that the septa, columella, and synapticula are built up of the same structural 

 elements or units (plate 10, fig. 65). These appear in sections as so many 

 distinctly round or polygonal bodies, fused together into a compact continuous 

 whole. Each unit of structure under low power reveals lighter and darker 

 parts arranged in a concentric manner, and under high power is found to be 

 made up of radiating calcareous fibers (plate n, fig. 66). The fibers seem 

 to start some little distance from a center, which is either clear or dark, and 

 practically homogeneous in structure ; further, they are seen to be arranged 

 in close concentric lamellae, and the boundary between each two layers is 

 often indicated by dark, finely granular particles, similar to those at the 

 centers of calcification. 



Employing the terminology of Miss Ogilvie, each structural unit is a 

 trabecula, here seen in transverse section. The radiating bundles of fibers 

 arefasactes, the light or dark centers from which the fibers radiate are centers 

 of calcification, and the concentric layers are growth lamella. 



In transverse sections of the septa of Siderastrea the centers of calcifica- 

 tion are some distance from one another, and each bears radiating calcareous 

 fibers all round ; but in the majority of corals the centers are closer, and 

 often appear in sec Lions of the septa as a more or less continuous dark line 

 or band along the septal axis the dark line of calcification. In this case the 

 separate fascicles or bundles of fibers are more distinct, and arranged in a 

 feather-like manner along each side of the dark line or layer. 



Much discussion has taken place with regard to the nature of the center 



