216 CEYLON PEARL OYSTER REPORT. 



In the upper parts of the corallum the synapticula form the only union between 

 adjacent septa, but a little distance lower down, and at some height above the 

 Aspidosiphon chamber, the interseptal loculi become largely filled up by a secondary 

 calcareous deposit, which, being of a dark colour from the presence of an abundant 

 brown pigment, stands in sharp contrast to the white septa and synapticula in a 

 rubbed down specimen. This deposit is crystalline in structure, and as it appears to 

 have been formed independently of and subsecpient to the original septa and 

 synapticula, it has all the characters of a " stereoplasm," i.e., a thickening on either 

 side of every septum filling up the interseptal loculi. It should be observed that 

 this deposit is most abundant and extends furthest towards the centre in the 

 interseptal loculi adjacent to the primary endosepta ; it is abundant, but does not 

 extend so far towards the centre in the loculi adjacent to the secondary endosepta ; 

 it is least abundant in the loculi adjacent to the tertiary endosepta (fig. II.). 



The structure of the septa is somewhat complicated, and different to anything 

 that has hitherto been carefully figured or described. Each septum is very thin at 

 its inner end and gradually thickens towards its outer or costal extremity. As is 

 shown in Plate III., fig. 12, its inner edge is produced into a number of irregularly 

 shaped teeth or paliform lobes, which merge into the columella, and the impossibility 

 of distinguishing between these paliform sej)tal offsets and true pali has been alluded to 

 above. The laces of the septa are covered with small spiniform granulations which 

 seem to radiate from a centre situated just within the lower end of the synapticulum. 

 These spiniform granules are shown by longitudinal, horizontal and tangential sections 

 to be the extremities of the radiating trabecular (using this term in Pratz's sense) of 

 which the septum is composed. Plate III., fig. 13, is a careful drawing of a horizontal 

 microscopical section through the middle of the corallum, above the level of the 

 Aspidosiphon chamber. The thin inner ends of the septa are shown to consist of a 

 series of trabecular, standing in a single line, but at intervals diverging from the 

 centre to end in a spiniform granule on the septal surface. Passing from the centre 

 towards the periphery, the thickening of the middle and outermost parts of each 

 septum is seen to be due to the multiplication of the trabecular, which no longer 

 form a single line but stand in rows, first two, then three, and finally, at the costal 

 extremity, five or six deep. It can be seen clearly enough in the horizontal section 

 that these trabecular radiate not only towards the central and the peripheral ends of 

 each septum, but also outwards towards the two septal surfaces, on which they 

 emerge as the spiniform granules already described. The septum, then, is formed of 

 a number of trabecular which radiate in all directions from an imaginary centre 

 situated low down and nearer to the inner than the outer (costal) margin of the 

 septum, and it is apparent that the centrifugal growth of the septum is effected by 

 the addition of new trabecular on the outside of those first formed, these new 

 trabecular being at first in single series, but in the more peripheral parts becoming 

 arranged in curved rows, two, three, and more abreast. The curved growth lines, 



