Mr. W. J. Sollas on the Genus Catagma. 357 



triradiate sections across the multiradiate spicules. It very 

 frequently happens that one ray of a multiradiate is directed 

 from the core outwards to the exterior of the fibre, beyond which 

 it projects echinately, having crossed the longitudinal acerate 

 spicules transversely in traversing the fibre. Sometimes a 

 row of two or three multiradiates may be seen lying side by 

 side, each echinating the fibre in this manner. 



In one or two instances the fibres radiating towards and 

 terminating against the surface of the sponge exhibit near 

 their extremities a few multiradiates (PI. XIV. fig. 20), which 

 are disposed with their long shafts parallel to the axis of the 

 fibre and their pointed terminal rays directed towards the ex- 

 terior of the sponge. More usually the reverse is the case, 

 however, and two of the rays lie transverse to the fibre, the 

 other pointing axially outward. Sometimes, again, a triradiate 

 form may be seen at the place of anastomosis of three fibres, 

 sending a ray along the axis of each fibre. 



The foregoing observations lead us to characterize the 

 skeleton of Catagma in the following manner : — Skeleton con- 

 sisting of an irregular network of fibres, now possessing a 

 calcareous composition, built up of spicules of two kinds : one 

 kind uniaxial, straight, simply curved or undulating, arranged 

 longitudinally in the exterior third of the fibre ; the other kind 

 multiradiate (3- and 4-radiate), with the adjacent rays of each 

 spicule making an angle of 120° with each other, or there- 

 abouts ; three of the rays occupying the interior or core of 

 the fibre, the fourth directed outwardly and penetrating the 

 layer of uniaxial spicules to echinate the exterior of the 

 fibre. 



Our next inquiry must be directed to determine the particular 

 group to which this structure belongs. We have five orders 

 to choose from, the Psammonemata, Chalinida, Echinonemata, 

 the Tetractinellid division of the Holorhaphidota, and the 

 Calcispongia. 



The lai-ge proportion of structureless calcite occurring in the 

 fibre seems to indicate the previous existence of kerataceous 

 cement, which has since become replaced by carbonate of 

 lime ; but even if kerataceous material is so indicated, which 

 is very doubtful, it cannot be taken as pointing to the Chali- 

 nida especially, since other groups of sponges, such as the 

 Rhaphidonemata, also contain this substance. The Chalinida 

 are, indeed, definitely excluded, not only by the presence of the 

 multiradiate spicules, but also by the forms of the uniaxial 

 spicules, which are quite inconsistent with Chalinid affinities. 



The structure of the fibre is also unlike that of any Psam- 

 monematous sponge ; but, in order to decide whether it might 



