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centre of the spicule, which is either formed by a thickening 

 of the silica of each ray at the point of junction ; or by the 

 giving off of slight buttresses from each ray which unite 

 together and form as it were a framework, of the figure of 

 a hollow octahedron, to inclose the junction of the canals 

 of each ray, in the centre of the spicule. To this latter 

 structure the term ' lantern » node has been applied. In addition 

 to these spicules of the .skeleton there are in the living 

 examples of the order very minute 'flesh' spicules of wonder- 

 ful variety and beauty of form, but in the fossil sponges 

 these have not been preserved. The skeleton of the Hexac- 

 tinellid sponges is either built up of spicules which are merely 

 connected together by the sarcode or else the spicules are 

 welded together by the extremity of their rays to form a 

 connected mesh-work with regular oval or oblong interspaces. 

 The hexactinellid sponges are also provided with an exterior 

 surface layer, composed of a delicate perforated siliceous 

 membrane which, in some instances at least, can be seen to 

 be formed by a modification of spicules similar to those of 

 the interior skeleton. In some species too the sponge was 

 anchored to the sea bottom b}^ elongated thread-like spicules 

 terminated by small anchor-.shaped heads. 



In this Horstead flint the hexactinellids are represented 

 bj' free spicules of various sizes; by small portions of the 

 spicular mesh of different species, also by fragments of the 

 surface membrane and by portions of the root- fibre and the 

 small terminal anchors. The free spicules appear to belong 

 to sponges of the group of the Lyssakina in which the 

 spicules were only connected by sarcode. Fragments of 

 the mesh-work of the group of the Dictyonina are not 

 infrequent and occur oftencr than in the case of the Lithistid 

 .sponges, which may be accounted for by the fact that the skeletal 

 mesh of the Hexactinellids is formed by a complete welding 

 together by silica of the adjacent spicular arms, and not, as 

 in the Lithistids, bj' a mere interlocking of their extremities. 



