96 CEYLON PEARL OYSTER REPORT. 



and is composed of stout oxea ; these are not arranged strictly radially, but are 

 inclined at various angles to the surface, with their outer extremities commonly 

 abutting at more or less acute angles against the stout bundles of the main skeleton 

 as the latter penetrate the cortex on their way to form the surface brushes. 



Spicules. (1.) Protrisenes (Plate IV., fig. 1, b) ; with long shaft, rather stout 

 distally but becoming setaceous proximally, and short, stout cladi of approximately 

 equal length. Shaft in measured specimen 478 millims. by 0'03G8 millim. (near 

 cladome) ; cladi, 0'129 millim. long. 



(2.) Anatrisenes (Plate IV., fig. 1, a) ; with long and very slender, setaceous shafts, 

 and short, stout, sharp-pointed cladi; shaft measured up to 4 '87 millims. by 

 0'0138 millim. (near cladome), with cladi 0'0736 millim. long. Very numerous. 



(3.) Oxea of the radiating bands (somal oxea) ; stout, aniso-actinate ; more or less 

 gradually sharp-pointed distally, but drawn out into long setaceous filaments 

 proximally ; size up to about 3'27 millims. by 0'046 millim. 



(4.) Cortical oxea (Plate IV., fig. 1, c.o.); stout, fusiform, aniso-actinate, sharp- 

 pointed at each end, the proximal end narrower than the distal, but not setaceous ; 

 commonly slightly curved; size about I'O millim. by 0'0368 millim. 



(5.) Sigrnata ; slender, 0'0166 millim. from bend to bend. 



The cortex of the type specimen measures, in places, as much as 1*8 millims. thick, 

 and is very sharply differentiated into two layers, of which the outer appears to be 

 usually somewhat thicker than the inner. The outer layer is excavated by very large 

 subdermal (intra-cortical) cavities (Plate IV., fig. 1, i.c.c.), which occupy nearly all 

 the space between the radiating spicule bundles of the main skeleton, with the thin 

 dermal membrane stretched over them. The dermal membrane (d.m.), and also the 

 deeper parts of the outer layer of the cortex, between the subdermal cavities, 

 contain numerous small brown pigment granules grouped in more or less spherical 

 cells. The inner layer of the cortex (f.c.) is densely fibrous and contains no pigment 

 granules ; it is strengthened by the special oxea above described, and the fibres lie, 

 for the most part at any rate, parallel to the surface. 



The flagellate chambers are about 0'02 millim. in diameter, spherical or oval, 

 eurypylous or with short, wide excurrent canaliculi. The choanosome is crowded 

 with large embryos (Plate IV., fig. 1, cmb.). 



In addition to the type above described, there is in the collection another very 

 small specimen (R.N. 204) of pear-shaped form, with a single apical vent. The 

 maximum diameter of this specimen is only about 8 '5 millims. In the structure of 

 the cortex and in the form and arrangement of the spicules, it agrees very well with 

 the type, but the megascleres are all much smaller. This difference, however, is 

 probably due to immaturity, the specimen being of very small size and containing no 

 embryos, while the type, as we have seen, is crowded with them. 



R.N. 193 (Station LVII., outside Dutch Modragam Paar, ll to 36 fathoms) ; 204 

 (Gulf of Manaar). 



