GEODIA MEDIA. 195 



are no clearly circumscribed pit-bearing areas as in the massive form, but pits 

 similar to those of the latter, are found singly and in groups, scattered irregu- 

 larly over the concave parts of the surface. Oscules or uniporal openings of 

 cortical canals are absent. 



The colour of the massive form is, in spirit, brownish white. Bowerbank's 

 dry specimen was pale buff-yellow. The surface of the elevations and the lower 

 side are lighter in colour than the depressed pit-bearing areas and the interior. 

 The digitate form is chrty white, a little darker in the interior. 



Various space-symbionts, some specimens of Donatia, a desmacidonid 

 sponge with exotyle spicules, Serpulae, and small composite ascidians, are at- 

 tached to the specimens of the massive form. 



The superficial part of the body forms a cortex composed of three layers : an 

 outer dermal layer (Plate 17, fig. 21a), a middle sterraster-armour la.yer (Plate 

 17, figs. 21c, 22c), and an inner fibrous layer (Plate 17, fig. 22e). The dermal 

 layer is free from sterrasters. In the pits above described it attains a consider- 

 able thickness and here it is excavated by extensive dermal lacunose canals. 

 Everywhere else it is but a thin membrane. The sterraster-armour la^^er, 

 which forms the largest part of the whole cortex, is filled with sterrasters. The 

 inner fibrous layer is thin and consists chiefly of paratangential fibres. It con- 

 tains only a few scattered sterrasters. The thickness of the cortex is very 

 different under different parts of the surface. In the pit-bearing depressions of 

 the massive form it is 0.5-1 mm., on the convex parts of the upper side about 1.5 

 mm. thick. On the lower side it is much thiimer. It attains the greatest tliick- 

 ness in the elevated borders surrounding the pit-bearing depressions, and is in 

 some parts of these 2.3 mm. thick, a great part of the whole border consisting 

 of cortical tissue. In the digitate form the cortex is 0.5 mm. (under the con- 

 cave parts of the surface), 1.2 mm. (under the convex parts of the surface) 

 thick. 



Canal-system. The pits congregated in the depressed areas of the surface 

 of the massive form and scattered over the concave parts of the surface of the 

 digitate form are covered by pore-sieves, which appear as nets of slender strands 

 with broad oval meshes (pores). In many places these pore-sieves (nets) consist 

 of a primary network of stouter, pigmented strands, in the meshes of which a 

 secondary network of more slender, mostly unpigmented strands is spread out. 

 The meshes of the primary network are 80-100 fi, those of the secondary network, 

 the pores proper, 15-30 fi wide. Where primary and secondary nets cannot be 

 distinguished the pores are also 15-30 fi wide. In the depressed areas of the 



