SIDONOPS ANGULATA. 25 



is rough and shagreened. On the sides, this roughness is due to the presence 

 of shallow pits, separated by a network of ridges somewhat raised at their junc- 

 tions; on the upper face it is due to numerous wart-like protuberances varying 

 in size and about 1 mm. apart. There are no larger oscules. On some of the 

 more protected parts of the surface a spicule-fur, up to 5 mm. high, is observed. 

 The specimen of var. microana (Plate 12, fig. 18) is irregularly spherical, 49 mm. 

 broad and 40 mm. high. The lower part of the body is somewhat drawn out to 

 form a peduncle, 30 mm. broad and 15 mm. thick, which is attached to a coral. 

 The upper side is flattened. The greater part of the surface is covered by a 

 dense spicule-fur which is in places 5.5 mm. high (Plate 13, fig. 21b). The 

 specimen of var. orthotriaena is a fragment of an irregular lobose mass. It 

 measures 33 mm. in length, 23 mm. in breadth, and 14 mm. in thickness. The 

 surface is quite smooth and without larger oscules. In places there are rem- 

 nants of a spicule-fur. A few insignificant sjanbionts, chiefly small crusts of 

 calcareous and monoaxonid silicious sponges, are attached to the surface of all 

 the specimens. The specimen of var. microana bears a dense growth of diatoms 

 on its surface. 



The colour, in spirit, is yellowish in the interior and white to reddish or 

 purplish brown on the surface. The lobose specimen of var. megana is quite 

 white on one side and has a reddish brown tinge on the other. The massive 

 specimen of this variety is partly yellowish, partly reddish white on the sides 

 (below) and reddish brown on the upper face (above). This colour is not uni- 

 form, some parts of the upper face being considerably darker than others. 

 Variety microana is dirty white below and purplish brown above, var. ortho- 

 triaena brownish white. 



The superficial part of the body is differentiated to form a cortex composed 

 of a thin outer dermal layer (Plate 14, fig. 20a), a central sterraster-armour 

 layer (Plate 13, fig. 21a, 25a) 0.7-1 mm. thick, and an inner fibrous layer (Plate 

 14, fig. 22a) excavated by subcortical cavities. 



Numerous granular cells, extended paratangentially and measuring 12-18 /< 

 by about 7 fi, lie in the dermal membrane just below the surface (Plate 14, 

 fig. 20b). Below these, between them and the most distal sterrasters of the 

 armour, slender fibres extend paratangentially. Those adjacent to the pores 

 are circularly bent and surround the pores sphincter-fashion. These fibres are 

 strongly stained with haematoxylin but only slightly with azure. In the choano- 

 some of the massive specimen of var. megana numerous irregularly polyedric 

 spaces, 40-70 /( in diameter, occupied by dense masses of small cells (Plate 15, 



