Sponges from South Australia. 439 



wards from the surface of the cloaca, this becomes particuhirly 

 striking and beautifid at the base of the peristome, where the 

 arms are not only expanded ahiiost perpendicuhxrly across the 

 lower ends of the spicules of the peristome, like cross bars in 

 a palisading", but, to still further extend their use, are abso- 

 lutely flattened vertically, while the shaft remains more or less 

 aborted, thin, round, and directed backwards in a line with 

 the spicules in the layer of the peristome. 



The quadriradiates, on the other hand, do not differ from 

 tile triradiates except in the addition of what is termed a 

 fourth arm ; but inasmuch as this is for the most part different 

 in form from any of the rest, it has been termed by Dr. 

 Bowerbank the " spiculum " or '' spiculated ray" (Mon. Brit. 

 Spong. vol. i. p. 241, pi. iv. figs. .85 and SG) ; we shall call it 

 the " fourth ray." It may be larger or smaller, longer or 

 shorter than either of the other rays, curved or straight, simple 

 or ensiform, according to the species and its position in that 

 species, situated perpendicular to the rest of the rays or in- 

 clined forwards in the sagittal forms. In the body of the 

 cloaca, where these spicules constitute a characteristic feature, 

 the fourth arm, which projects into the interior, is perpen- 

 dicular to the other three (which are fixed in the surface 

 of this cavity), with the curve always directed towards the 

 mouth, and in this form they may be traced more or less 

 into the canals leading into the cloaca. On the other hand, 

 when the quadriradiate is large or constitutes, from its size 

 and predominance, the greater part of the spiculation, as in 

 Leucoma Jo/ntstoiii'i, Carter {Leucandra Johnstoniij H.), it may 

 be more or less like the rest ; but under these circumstances this 

 spicule may in situ be frequently distinguished from the tri- 

 radiate by presenting a dark triangular space in the centre of 

 the other three rays, whose angles are coincident with those 

 of the triradiate portion, and whose darkness arises from the 

 rays of light at this part passing through the surface instead 

 of being reflected from it; while in the triradiate there is no 

 " dark space " visible unless the spicule be viewed laterally, 

 when a similar thing happens through the position of the 

 third ray; but its shape is quadrangular and more or less 

 concave at the sides, like an hour-glass ; hence, as these 

 spicules lie in situ on the surface in L. Johnstonii &c., where 

 the fourth ray is directed inwards and the triradiate portion 

 lies flat on the surface, the " dark space " is only seen in the 

 former. 



Among the triradiates may be mentioned in particular a 

 form very much like a " tuning-fork," in which the arms 

 are projected forwards almost parallel to each other and closely 



30* 



