- 6 3 - - 



slender tapering arms, spread out in a plane at right angles 

 to the shaft and united together near the shaft by a solid 

 siliceous membrane. The angle embraced by the four arms 

 varies between 80 and 135 in different specimens. Only 

 traces of canals are shown in the Horstead specimens but in 

 similar spicules from Coesfeld, they are present both in the 

 rays and shaft, and unite at the junction of these. I have 

 not yet met with a complete specimen, but the arms appear 

 to have been of equal length ; the longest which I have seen 

 is 0,9 mm. The longest shaft met with is 0,787 mm. with a 

 thickness of 0.056 mm. In all the specimens the connecting 

 membrane between the arms is only developed but a short 

 distance from the shaft. The spicules are somewhat rare. 



No spicule at all resembling this has been met with 

 in any recent sponge and its affinities are very doubtful. 

 The number of the rays remove it from relationship with 

 Tetractinellid sponges as also with surface spicules of the 

 known Lithistids. It is possible however that it may be the 

 surface spicule of a Lithistid sponge as there is a great 

 diversity known to prevail in the outer spicules of this group. 

 This spicule was first noticed in the Chalk of the North of 

 Ireland (Wright : op. cit. p. 90, Plate III, figs. I a, b.) and 

 Dr. Bowerbank expresses the opinion that it is 'from the 

 expansile dermal system of a siliceo-fibrous sponge' . Professor 

 Zittel has also figured several examples of the same spicule 

 from the Upper Chalk Formation of Coesfeld (Ueber Coelop. 

 p. 46 PL V. fig. 4750). 



Order Hexactinellidae, Oscar Schmidt. 



The sponges of this order are built up of spicules com- 

 posed of six rays which form three axes crossing each other 

 at right angles. In each ray there is a distinct canal which 

 unites with those of the other rays in the centre of the 

 spicule. There is an enlargement, in many instances, in the 



