REPORT ON THE DEEP-SEA KERATOSA. 



63 



described ; it differs from the others mainly in the composition of the skeleton, in which 

 the interwoven bundles of spongin-fibrillse predominate, whereas the inorganic xenophya 

 are less numerous. The fibrillse are much more fully developed than in the other species, 

 and form strong bundles, the smaller of which are composed of ten to twenty, the larger 

 of thirty to fifty or more, parallel fibrillse. These are nearly equal in size, of medium 

 thickness, their diameter being usually 0'005 mm. on an average (0 - 002 to 0*008 mm.). 

 Their yellow colour effects the brown tint of the sponge, which is much darker than in 

 the other species. The bundles of fibrillse are interwoven and cross in all directions, 

 forming an elastic framework, in the smaller meshes of which are imbedded the xenophya, 

 in the larger the canal-system and its flagello-chambers (PI. II. fig. 2). The xenophya 

 are partly Radiolarian shells, partly fragments of Globigerina shells, the former usually 

 much more numerous. 



Xenophya. — The foreign bodies which compose the pseudo-skeleton are in Stanno- 

 <pliyllum zonarium relatively less numerous than in the four other species of the genus ; 

 they are for the most part siliceous shells of Radiolaria, mainly in the distal portion of 

 the leaf, while in the basal portion fragments of Globigerina shells and fine inorganic 

 particles are intermingled. 



External Form. — The flabelliform body of Stannophylhim zonarium is easily 

 distinguished from all the other species externally in the softly coriaceous shape of the 

 thick roundish leaf, the two faces of which exhibit sharp concentric zones, but no ribs. 

 The basal pedicle is flat and thin, tapering towards the basal insertion, 10 to 30 mm. 

 long., 1 to 5 mm. thick. The flat leaf is 40 to 60 mm. in diameter, and is sometimes 

 subcircular, at other times reniform, with a flat basal excision. Its thickness is between 

 1 and 3 mm., usually 1*5 to 2 mm. The colour is deep brown in the wet state, yellow- 

 brown in the dry state. The thick rounded distal margin is integral and not lobate. 

 The two parallel surfaces of the thick leaf exhibit a most striking zonary structure. 

 Numerous concentric deep furrows, which run parallel to the semicircular distal margin, 

 divide both faces into zones or bands of subequal breadth (between 3 and 5 mm., usually 

 4 mm.). The proximal part of the concentric bands is somewhat thicker than the distal, 

 so that they exhibit a slight imbrication. This zonary structure presents a striking- 

 similarity to that of two other flabelliform but widely remote organisms, viz., Flustra 

 foliacea (Bryozoa) and Zonaria pavonia, Ag. Padina pavonia, Grev. (Fucacese 

 Dictyotese) ; even the breadth of the concentric zones is usually about the sam„. 



The consistence of this species is much denser and more elastic than in any of the 

 other species of the genus, owing to the much stronger development of the spongin- 

 fibrillse and the smaller quantity of imbedded xenophya. The thin dermal membrane 

 is denser and more coherent than in the others, and the medullar substance is also more 

 consistent. The dermal pores are very small and regular. 



