20(5 CEYLON PEARL OYSTEE REPORT. 



quite confined to one surface of each lamella. The vents are compound, each con- 

 sisting of an aggregation of several smaller ones, the entire group only from 1 millim. 

 to 2 millims. in diameter. 



The main skeleton (Plate XV., fig. 5) is a rectangularly meshed network of very 

 distinct primary and secondary fibres, the primaries running vertically to the surface 

 and the secondaries crossing them more or less at right angles. The primary fibres 

 average about 0*049 millim. in diameter, and the secondaries about half as much. 

 The outermost secondary fibres form a well-developed dermal or sub-dermal skeleton 

 (Plate XV., fig. 5, d.s. ; fig. 6), in the form of an irregular network with fairly wide 

 polygonal meshes, the fibres averaging about 0'02 millim. in diameter. 



There are many more minute conuli on the surface than there are of the stout 

 primary fibres, and those which do not contain the apices of such fibres are supported 

 by short fibres which spring vertically from the tangential fibres of the dermal 

 skeleton (Plate XV., figs. 5, 8). 



All the fibres of the skeleton are composed of pale-coloured spongin without any 

 foreign enclosures. They are not distinctly " pithed " except at the growing apices 

 (Plate XV., fig. 7), where the usual thimble-shaped layers of spongin are added one 

 on top of the other, as in other Aplysillidse, but even here the " pith " is not distinctly 

 differentiated. 



The ectosome forms a thin dermal membrane containing the inhalant pores, and 

 the choanosome is very delicate and gelatinous. The canal-system agrees closely with 

 that of Aplysilla, as described and figured by SCHULZE (70). It is lacunar, and the 

 flagellate chambers are sac-shaped and large, averaging when full-grown about 

 0'07 millim. in diameter. They are not placed very close together and they open 

 directly into the excurrent lacunae, without special exhalant canaliculi. 



Pv.N. 71, 16lA (Pearl banks, Gulf of Manaar not uncommon). 



Megalopastas pulvillus, n. sp. Plate XV., fig. 3. 



The single specimen has the form of a small flattened cushion, slightly convex 

 above and (has been) attached by a broad flat base below. The outline of the 

 specimen is irregularly rounded. The upper surface bears several small, compound 

 vents, each about 2 millims. in total diameter ; it also appears granular from the 

 presence of numerous minute, slender, sharp -pointed conuli containing the ends of the 

 primary fibres. The inhalant pores are conspicuous under the microscope in small 

 groups in the thin, translucent dermal membrane. Texture (in spirit) firm, but 

 compressible and resilient ; colour, pale yellowish -grey. Diameter of specimen about 

 18 millims. ; thickness in the middle about 5 millims. 



The skeleton (Plate XV., fig. 3) is a partly rectangular- and partly polygonal- 

 meshed network of pale amber- coloured horny fibre, in which the primary fibres are 

 very clearly differentiated, radiating towards the surface and terminating in long, 

 slender apices in the surface conuli. The primary fibres not infrequently branch, 



