114 
STAUROCALYPTUS HAMATUS. 
the dermal spicules of the sponge. Spicules which might be considered as gas- 
trals were not observed. 
The choanosomal and prostal rhabds (Plate 16, figs. 25-38, 39a, b; Plate 18, 
fig. 13) are usually more or less curved, and exceedingly variable in size. They 
are 0.67-13 mm. long, and 5-175 n thick at the thickest point. The rhabds 
under 3 mm. in length are less than 50 n thick, those 3-9 mm. in length are 40- 
100 /x thick, those over 9 mm. in length, usually 100-160 n. Although there is, 
as this shows, on the whole, a certain correlation between thickness and length, 
the proportion between these two dimensions is nevertheless very far from being 
constant and varies between 50 to 1 and 122 to 1. The thickest point of the 
rhabd may be situated at or near the middle of its length (Plate 16, figs. 29, 39a), 
or it may be more (Plate 16, fig. 30) or less (Plate 16, fig. 34) approximated to 
one of the ends. A tyle is met with only exceptionally. It is, when present, in 
the small rhabds 4-6 m more in transverse diameter than the adjacent parts of 
the spicule, and may be situated near the middle or nearer one end. Occasion- 
ally it lies quite terminally, in which case the spicule appears as a tylostyle. In 
the large rhabds the axial thread is usually somewhat thickened (Plate 16, 
fig. 36) at several points, but an axial cross can only rarely be made out. In the 
small rhabds an axial cross can generally be found. When a tyle is developed 
the axial cross generally lies in its centre. In the large rhabds the two rays taper 
towards the end and are usually abruptly and bluntly pointed (Plate 16, figs. 27, 
33, 35, 37), rarely rounded or sharp-pointed. In these spicules the ends are 
55 -3 as thick as the thickest portion of the middle-part. In the small rhabds the 
ends are cylindroconic or quite cylindrical and terminally either abruptly and 
bluntly pointed, like the ends of the larger rhabds (Plate 16, fig. 26), or more 
rounded (Plate 16, figs. 25, 27). In these spicules the ends are from half as thick 
to quite as thick as, or even slightly thicker than, the thickest portion of the 
middle-part. In the rhabds in which the thickest part lies near one end, this end 
is conic and stout (Plate 16, fig. 33), the other being cylindrical and slender 
(Plate 16, fig. 31). 
The whole of the rhabd, with the exception of the two ends, is smooth. 
The ends are covered with broad, conic, vertically arising spines 0.5-1 /x, rarely 
2 /x long. The terminal spiny region is 40-230 /x long and passes, as the spines 
become scarcer and lower, gradually into the smooth middle-part of the spicule. 
In some of the rhabds an abrupt step-like attenuation occurs at a shorter 
or longer distance from one of the ends. Of other rhabd-irregularities noticed 
I mention slight transverse grooves which give to the contour an indented ap- 
pearance. As the figure (Plate 18, fig. 13) of such a spicule clearly shows, these 
