SEA-BOTTOMS AND CALCKETKS. 



15M 



origin 



chitinous patches are often associated with the calcite rods. Their nature am 

 will he discussed in the next example from the middle portion of the Paar. 



Middle Par t. Nine specimens, labelled " half way up Periya Paar," consisted 

 of loose porous calcretes, white in colour and heavily encrusted, and a large Chank 

 shell rilled with calcareous sand and coated with Polyzoa, Nullipore, calcareous worm 

 tubes, byssus of pearl oysters, and sponges. Sections of the calcretes were obtained 

 showing the encrusting organisms in situ. This was a matter of some difficulty, 

 owing to the friable nature of the material, but on soaking in thin balsam and 

 hardening, slices were cut and rubbed down on carborundum blocks. 



On examining a thin slice with recent colonies on the outside, the surface layers 

 showed Polyzoan cells arranged in parallel rows, two or three deep, with the avicularia, 

 vibracula, and opercula in position. The irregular surfaces served to entrap and 

 retain sand grains and Foraminifera drifting over them. Naturally, only those were 

 held which fitted closely into the spaces provided, and thus a sifting action took place. 

 Occasionally two small grains would fill the space in place of one of larger dimensions. 

 Below the surface layer other cells were seen, nearly all of which contained a grain. 

 The base and side walls were perfect, but the 

 top wall had broken down to admit the sand 

 grain, carrying with it the chitinous oper- 

 culum. Proceeding towards the centre, suc- 

 ceeding layers have less prominent walls, 

 owing to secondary calcite growing from 

 them in tiny scalenohedra towards the interior 

 of the cell, and finally we reach a stage when 

 a thin dark line marking the junction of 

 adjacent walls, the roughly linear arrange- 

 ment of uniform grains, and the occasional 

 remains of chitinous opercula are all that 

 remain to indicate the former presence of 

 Polyzoa. Cells which escape being occupied 

 by sand grains become filled with secondary 

 calcite showing a radial structure (see fig. 2). 



Nullipores sometimes alternate with Poly- 

 zoa and enclose grains, but it is not difficult 

 to distinguish between the two even when 

 structure has been lost. The grains cemented 

 by Nullipore are sporadic in their distri- 

 bution, there is no sifting into grains of uniform size, and when they appear in 

 linear series they are mostly radial, not concentric. The action can be seen in masses 

 of Lithothamnion, which send out club-shaped extensions from their surfaces. The 

 spaces between the branches are often filled with loosely-held sand grains, and 



x 



Fig. 2. Section of C'alcrete with recent Polyzoa 

 on surface, from Periya Paar Kerrai. The 

 two upper layers represent sections of 

 Polyzoon cells, and the lower part shows 

 sand grains (shaded) in a calcareous matrix 

 and fragments of the walls and opercula of 

 the Polyzoa. x 25. 



