44 SIDERASTREA RADIANS. 



diameter. The latter are very variable in size in the columella (plate lo, 

 fig. 65). Sometimes two centers of calcification, diverging from one another 

 in the median plane, are present in one concentric system, and the trabeciila 

 then appears double and is elongated in outline. Fusion between adjacent 

 trabeculas is usually indicated bj' a narrow, dark line. 



A synapticulum may have either a separate center of calcification (true 

 synapticulum) or be formed from the fascicles of trabeculse in adjacent septa 

 which extend beyond the septa until they meet in the middle of the inter- 

 septal space (false synapticulum, p. 52). Bundles of fibers or fascicles from 

 individual centers also pass into the granulations over each face of the septa. 



The growth lamellae in the individual trabeculae are not always clearly 

 indicated, but sometimes they come out with remarkable distinctness, having 

 a black, finely granular margin (plate 11, figs. 66, 68). Around the centers 

 of calcification they are strictly concentric, but towards the edges of the septa 

 they more nearly follow the septal outline. In transverse sections they are 

 distant from one another about 0.0025 mm. The black, finely granular 

 margin is probably organic residue of the same character as that at the 

 centers of the trabeculae. 



A thin vertical section of part of two contiguous septa is represented on 

 plate II, fig. 67, and displays a series of elongated bands. One of these, the 

 continuous median band, represents the thecal wall, and from it the con- 

 stituent parts of each septum diverge. The darker circular or oval areas also 

 seen in the section are the transversely cut surfaces of synapticula, and each 

 originates in connection with a band. 



The broad radiating bands are the septal trabeculae as they appear in 

 longitudinal section. Where the section passes through the middle of a 

 trabecula a row of close centers of calcification is revealed, and fascicles of 

 fibers pass from them obliquely upwards on both sides. Peripherally the 

 fibers of adjacent trabeculse meet all the way in a distinct thin line, so that a 

 compact septum is the result. Fresh trabeculae are seen to be inserted at 

 intervals between the old. The trabeculse are continued upward to the septal 

 margin and there constitute the terminal teeth, each tooth corresponding with 

 a single trabecula, but the preparation from which plate 11, fig. 67, is taken 

 was not sufiiciently wide to show the teeth. They vary but slightly in width, 

 being about 0.125 mm. across towards the margin of the septum. From the 

 inclined arrangement of the trabeculae it is obvious that a transverse section 

 of the septum will cut only the more peripheral at right angles ; the others 

 will be represented in varying degrees of obliquity. 



Each separate dark area along the middle of a trabecula represents a 



