TETRAXONIDA. 15 



discasters vary somewhat, the apparently typical form being 64 /a long and S9 fx, 

 broad, with a small spike at the base, and a still smaller one at the apex ; sometimes 

 neither the basal nor apical spines are present, and the spicule resembles more nearly 

 the discaster of Latrunculia brevis Ridley and Dendy. There are two small basal 

 whorls of spines ; above them is a plano-convex whorl with denticulate edges and 

 deeply cleft along 3-5 radii; then follow two smaller whorls pointing upwards. 



The smooth styles are fusiform, with tornote point, 557 /a long, 6-5 ;u, in diameter 

 at the head, and 10 '5 in diameter at the centre. 



The specimen was dredged West of Balleney Island in 254 fms. 



Family- Polymastidae, Vosmaer. 

 Polymastia invaginata. 

 (Plate XIL, fig. IB, and Plate XIV., figs. 5-1 5a.) 

 Polymastia invaginata Kirkpatrick (10a. p. 271). 



Sponge hemispherical, free or attached, covered with a thick pile of pointed 

 spicules. With one large oscular papilla usually completely invaginated, so that the 

 summit of the oscule is on a level with, or below the general surface. Under surface 

 with a fleshy basal pad. 



Pores in longitudinal meridional groups in the cortex. 



Colour^ in spirit pale yellow above, and often gray and semi-transparent on the 

 under surface in free specimens. 



Consistence dense and firm. 



Flagellated chambers spherical, 30 • 5 ju in diameter, diplodal (XIV. 7). 



Skeleton. Choanosomal, formed of fibres curving upwards from the base to the 

 periphery, penetrating the cortex and forming the thick surface pile ; with stellate 

 clusters of small tyles between the fibres. 



Cortical skeleton formed of a dense layer of vertical tyles of various lengths 

 embedded in a tough, fibrous layer from "5 to 1'25 mm. thick. 



Basal skeleton consisting of spicules transversely arranged, and crossing each other 

 in an irregular manner. 



Spicules. Large, smooth, slightly curved styles, or occasionally strongyles, 

 2240 X 40/i. 



Cortical tyles, with small spheroidal head, short neck, fusiform straight shaft, 

 varying in length from 140 to 350 jx, and in thickness from 12 to 19 /x. A few very 

 slender styles scattered in the choanosome, 70 X 6 /x, with head and neck making an 

 angle with the shaft. Some medium-sized cortical tyles in the oscular papilla with long, 

 oval heads. Tyles of the stellate clusters, slender, with the head making an angle 

 with the shaft, 20T) x 15 /x. 



There are fourteen large specimens and five very small ones of this species. The 

 largest is 8 cm. in diameter and 4 ' 5 cm. high ; and the smallest, which is triangular, 

 VOL. IV. 2 



