Genus of Fossil HexactinelUd Sponges. 13 



terminations (PI. III. fig. 2) ; but now and then these termi- 

 nations are wanting, and the quadrilateral meshes from which 

 they spring lie level or nearly level with the surface. From the 

 spinose ends, or from the quadrilateral meshes, an exceedingly 

 fine network of delicate, glassy, pullulating fibrelets is given 

 off, which fills up the interstices of the secondary rete 

 (PI. III. fig. 4, PI. IV. figs. 4, 5, 6, 7, PI. V. fig. 4) ; 

 frequently it is wholly irregular, but in numerous instances 

 exjiibits the true sexradiate arrangement. Its meshes and 

 fibrelets vary in size, the average measurement from node to 

 node being y^ to ji^ inch, and the diameter of the fibres 

 TFOir to -goVo. Thus the latter are, as a rule, not appreciably 

 thicker than the spines of the secondary rete : and this suggests 

 that some of these spines may be, after all, nothing but the 

 attached parts of fibrelets, which have been broken off or dis- 

 solved away ; and often a series of gradational forms can be 

 traced, proving that some are of this nature ; but many, from 

 their smooth sides, regular conical form, and abundance in 

 places free from fibrelets, must, as we have already considered, 

 be true spines. 



From the minuteness and proximity of the sexradiates one 

 would conclude that they have been coated merely with a thin 

 film of siliceous material, or are only soldered together at 

 their ends; and the same characters would also lead us to infer 

 that they do not afterwards come to form a part of the interior 

 skeleton, but remain as a surface-coating, which must be 

 regarded as an aftergrowth creeping over the oscules of the 

 anterior plate, as this becomes overgrown by the posterior 

 mass behind. 



Though this network is in general collected only about the 

 ends of the radii from the secondary rete, beneath or between 

 the meshes of this rete, it yet also happens occasionally, espe- 

 cially near the base, that it accumulates in patches to a much 

 greater extent, burying up the network below, so as to com- 

 pletely conceal it from sight (PI. IV. fig. 4), and forming a 

 low but distinct mound above the general surface, and even, 

 in one case, producing a series of rounded ridges (PI. I. 

 fig. 2, PI. II. fig. 2, ?•) which pass straight across the 

 anterior face of the oscular plate, horizontally from one side to 

 the other. 



The superficial network, where it covers up the oscules, 

 descends some distance into the excurrent canals, as may be 

 well seen by breaking a specimen across the oscular plate, 

 etching the fractured surface, and then examining it by 

 reflected light. The skeleton-fibres, with their projecting 

 spines, are then exposed ; the superficial network is seen 



