54 DK. G. J. HINJDE ON CALCISPONGES [Feb. 1 900, 



The union is not of a fixed character like that of the mesh-spicules 

 of the sponge, for by slight pressure the individual spicules 

 may be separated without difficulty uninjured. The mesh is 

 close and intricate, so that it is not easy to trace single spicules, 

 but in some instances there seems to be a definite orientation 

 whereby the pointed apical or free rays are directed upward (PL IV, 

 figs. 2 & 3). In the case of the transverse platforms, the basal spicules 

 are frequently attached to the much larger mesh-spicules as well as 

 one to another. 



Similar transverse layers or floors, indicating periodic stages of 

 growth, have been noticed in other fossil calcisponges, and more 

 particularly in species of Lymnorella? but hitherto their spicular 

 structure has not been recognizable. 



Skeletal Mesh. 



The main skeleton of the sponge is very firm and resistant ; when 

 seen under the microscope by reflected light, it appears to consist of 

 stout, cylindrical, continuously anastomosing fibres of a rough, dull, 

 glassy aspect, forming a close meshwork with oval, rounded, and 

 elongated interspaces, and also enclosing definite canals. From the 

 surface of the fibres, at intervals, sharply pointed spurs extend into 

 the interspaces, and these are also partly occupied by loose wisps 

 of simple rod-like spicules. 



The structural details of the mesh-fibres can be seen only in thin 

 microscopic sections mounted in Canada balsam. These show that 

 the fibres consist of four-rayed spicules in which the apical ray is 

 relatively very elongate, tapering to an acute point, and armed 

 Literally with a few stout, horizontally projecting prickles (PL IV, 

 figs. 4-8). The facial rays of the spicule, on the other hand, are 

 short, robust, straight, or curved in a direction opposite to that of the 

 apical ray ; they do not taper as a rule, but terminate in flattened or 

 concave expansions of a character similar to those of the spicules of 

 the basal layer, described above. Exceptionally, however, one of the 

 facial rays of a spicule is extended and tapers to a blunt end. The 

 apical rays in these mesh-spicules range from 0*2 to 0*35 mm. 

 in length, while the facial rays range from 0*1 to 0*18 mm., 

 and they are about O05 mm. in thickness at the base. Neither in 

 these comparatively large mesh-spicules, nor in any of the smaller 

 spicules of this sponge, though their condition of preservation is so 

 perfect, is there any trace of an axial canal : in this respect they 

 differ markedly from fossil siliceous spicules. 



These mesh-spicules are united to form the fibres by the firm 

 apposition and close fitting of the expanded terminations of their 

 facial rays to the surfaces of adjoining spicules in much the same 

 manner as the spicules of the basal layer, but they are, as a rule, 

 oriented with much greater regularity, for the free tapering apical 

 rays are disposed so that they point in a generally vertical direction 

 towards the upper surface of the sponge (PL IV, figs. 4, 5, 6). As 



1 Monogr. Pabeont. Soc. ' Brit. Foss. Spong.' pt. iii (1893) pp. 236, 237. 



