TETRAXONIDA. 17 



canal in the cortex ; the mouth or pore of the pore canal guarded by a ring of 

 radiating rortinil i : Mil. :{). Flagellated chamlwrs diplodal (XIII. 7). 



Skeleton formed mainly of radiating fibres composed of styles, with diverging 

 brushes of spherostyles near the surface. Cortex with a surface layer of densely 

 picked tufts of small vertical tyles, and a sub-cortical layer of tangential styles 

 and tyles. 



Spicules. Spherostyles (XII. 8-12) 8 mm. in length by 30 in diameter in 

 the middle, and 1 4 p. in the region below the distal knob ; distal knob 28 in diameter, 

 hemispherical, with granular surface and with a few square teeth or serrations on 

 the edge. 



Styles straight, fusiform, blunt-pointed, 2*8 mm. in length, 41/i in diameter in 

 the middle, 23 /i in diameter at the rounded end. 



Cortical tyles (XII. 15, 15a), curved, 146/i long, head, 3 '25 n in diameter ; slender 

 neck, 2'75/* thick with broad oar-blade-like shaft, circular in section, 7/i thick. 



Styles of lower cortical tangential layer (XII. 7), also in choanosome, 900 x 20 /*. 

 Tyles of the same layer, nearly straight, 270 n long ; with head 7 n in diameter, 

 and relatively thick neck 6'Sn in diameter (XII. 16-16A). 



Slender curved tyles, 460 x 10 /*, scattered in choanosome (XII. 13). 



Young specimens are oval with one long closed papilla ; the bundles of divergent 

 exotyles are more or less separate and distinct, and the distal knobs retained and not 

 broken off ; the radial bundles of fibres radiate out from a point below the centre of 

 the specimen. 



There are five large specimens, of which three encrust rocks and two are free, and 

 six very small ones, all of which are free. One of the large free ones (XII. 5) is a 

 globular fluffy ball 6 cm. in diameter, with a few stumpy, much contracted papilla; ; 

 another free specimen is dome-shaped, with a flat, fleshy base like a thin pad. The 

 typical shape is that of a dome, with the skeletal fibres radiating upwards and 

 outwards from the base to the surface. The spheroidal form results from the growing 

 round of the edges till they meet ; accordingly a section shows a central core with 

 cortical microtyles still present, but displaced, and with main fibres radiating out all 

 round the core. 



The largest encrusting specimen (XII. 1), which is growing on a boulder of black 

 volcanic rock, is 10 cm. in diameter and about 3 cm. in height, the papilla;, about 

 twenty in number, rising to an additional height of 3-4 cm. The papilla: are coated 

 with the thick pile of microtyles, but there are no exotyles. A few of the papillae 

 have large terminal orifices with apparently cicatrised edges, as if the breach had arisen 

 from a bursting away of the summit from pressure from within the papilla ; these 

 papillae, at any rate, are oscular ; but so also, probably, are some of the closed papillae, 

 which have irregular cribriform patches around the summit or along one side of the 

 papilla. The same difficulty of deciding on the function of these organs occurs also in 

 the case of species of Polymastia, 



2 c 2 



