STAUROCALYPTUS HAMATUS. 
113 
Its lower, concave side fits the dorsal side of a crustacean, apparently a species 
of Dicranodromia, which firmly holds the sponge on its back by the dorsally 
directed, last pair of thoracic extremities. In its original position the sponge 
completely covered the Dicranodromia dorsally (Plate 18, fig. 14). Seen from 
above (Plate 18, fig. 5) or below (Plate 18, fig. 6) the sponge appears oval in 
outline, with a protuberance at one end. It is 35 mm. long and 28 mm. broad. 
The wall of the inverted cup, formed by it, is about 3 mm. thick. Scattered 
pores are observed both on the free upper convex side and the lower concave side 
which rested on the back of the Dicranodromia. Those of the upper side are 
mostly oval, with a maximum measurement of 1 mm. in length and 0.5 in breadth. 
Those of the lower side are relatively broader, more nearly circular, and reach 
1.5 mm. in diameter. Large prost.al rhabds protrude both from the upper and 
the lower side. 
The colour in spirit is light brown. 
General structure. I found a few remnants of a dermal membrane both on 
the concave, lower, and the marginal part of the convex, upper side. Of a 
gastral membrane no trace could be detected. The remnants of the soft parts 
in the interior indicate that the sponge has sac-shaped flagellate chambers, 80- 
100 yu long and 50-70 n broad. 
Skeleton. Spicule-bundles, 40-200 m thick, traverse the sponge. These 
bundles appear to be most numerous just below the lower, concave face of the 
sponge, where they extend chiefly paratangentially. They are composed of 
rhabds — of small rhabds only, or of a large rhabcl accompanied and more or less 
enveloped by numerous, comital, small rhabds. Besides the rhabds forming 
the bundles, isolated rhabds also occur in large numbers. Oxyhexasters, hemi- 
oxyhexasters, and oxyhexactines with straight rays and end-rays, oxyhexactines 
with terminally curved, hook-like rays, and discoctasters of various size are very 
numerous. The last appear to be much more frequent in the interior than near 
the surface of the sponge. Small discohexasters, and hemioxyhexasters with 
rays, either all hook-like or partly hook-like and partly straight, are met with 
in smaller numbers. Hypodermal pentactines and a few triactine megascleres 
occur at, or just below, the surface. On those parts of the surface where rem- 
nants of the dermal membrane are left, spiny rhabds are observed. Most of 
these are simple diactine rhabds. Some are centrotyle, and a few possess, 
besides the two properly developed rays, short rudiments of one or two further 
rays. These spicules and a few angular diactines and stauractines, similar in 
regard to size and spinulation, found in the spicule-preparations, I consider as 
