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 m thick at the thickest point. The rhabds 

 under 3 mm. in length are less than 50 m thick, those 3-9 mm. in length are 40- 

 100 fi thick, those over 9 mm. in length, usually 100-160 /i. 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 Ues 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 

 5^-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 wh(jle 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 m, rarely 

 2 IX long. The terminal spiny region is 40-230 ju 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 



