Vol. 56.] FROM THE EOCENE OF VICTORIA. 55 



the fibres are reticulate, these apical rays do not always coincide in 

 the same direction, and consequently their spur- like distal ends 

 frequently project beyond the surface of the fibres into the mesh- 

 interspaces. 



In addition to the close union of the mesh-spicules by the apposi- 

 tion of the terminal expansion of their facial rays, they are further 

 united and cemented together by the deposition of a calcitic layer, 

 which envelops the mesh-fibres and binds together their constituent 

 spicules in much the same manner as the spicules in the skeletal 

 mesh of the Dictyonine Hexactinellids are united by an envelope of 

 silica. In this calcisponge the common investment consists of a thin 

 pellicle of calcite, the outer surface of which is covered with numer- 

 ous minute blunted tubercles or acute prickles (PI. IV, figs. 4-8). 

 This pellicle does not seem to invest the fibres evenly throughout ; 

 it has a scabious appearance, as if formed in patches whose margins 

 are in irregular contact and sometimes overlap. In cases where mesh- 

 spicules stand out singly from the fibres they are not covered with 

 this pellicle, the terminal points of the apical rays which project 

 freely beyond the fibres are likewise free from it, and these points 

 are always smooth and glistening, in contrast to the whiter and less 

 transparent aspect of the portion of the spicules over which the 

 pellicle extends. 



In order to test the stability of the union of the spicules in the 

 fibres, small fragments of the mesh were treated for a short time in 

 dilute acetic acid until they were considerably reduced, and they 

 were then washed and subjected to pressure under a cover-glass, 

 with the result that the spicules generally gave no signs of 

 separation, but broke up irregularly instead of parting where the 

 junctions had originally taken place. In many cases the planes 

 of contact, where the terminal expansions had clasped round 

 adjoining spicules, could not be distinguished, and complete fusion 

 had been effected at the point of union. The elongated apical 

 rays, so conspicuous in the fibres, are also firmly welded to the 

 surfaces of other spicules in the fibres with which they may be 

 laterally in contact. In some exceptional instances, when the fibres 

 are broken up with a needle, a spicular ray will disconnect from 

 its original attachment without fracture (PI. IV, fig. 7), and in 

 this eventuality the ray has not been enveloped by the calcitic 

 pellicle. 



That the welding together of the spicules in the mesh-fibres 

 cannot be attributed to the results of fossilization is shown by the 

 fact that in the interspaces of the mesh are numerous minute 

 spicules intermingled, yet perfectly free from attachment; and, 

 further, the irregular spicules of the basal layer, though connected 

 by clasping in the same way as the mesh-spicules, are not fused 

 together, and readily separate without fracture. 



The disposition of the spicules in the mesh is most favourably 

 shown in vertical sections of the sponge (PL IV, figs. 4 & 5), while 

 in transverse sections the canals are exposed, and the mesh-fibres 

 appear to be closer and more confused than in the vertical sections 



