- 65 - 



An approximation to the affinities of these fragments of the 

 skeleton ma)- be made b\' a comparison of the regular or 

 irregular dispu>iti()n of the mesh, its dimensions and form, 

 and by the solid or hollow characters of the central axis of 

 the spicules. Reliable indications are also furnished in some 

 instances b)- the surface membrane, and the connection be- 

 tween this outer membrane and the interior skeleton is occa- 

 sionallj- shown by fragments in which a layer of the interior 

 skeleton spicules }et remains attached tu the surface membrane. 

 Unfortunately most of these hexactinellid fragments have been 

 injured b\' the fossilization ; the interior canals have been 

 obliterated, the --mall processes forming the lantern knots have 

 been worn through, and the spicular arms ha\e been so eroded 

 that it is difficult to tell definitel\- whether they were origi 

 nally smooth or spinous. 



Sub-order Dictyonina, Zittel. 



Genus Leptophragma, Zittel 1877. 



Leptophragma sp. 



(Plate V, 6g. 17). 



The disposition of the spicules of the mesh is irregular 

 so that the interspaces are either oblong or triangular. The 

 central node of the .spicules is solid and but slightl)' enlarged. 

 Average thickness of the arms of the .spicules 0,078 mm., 

 length of the more regular openings of the mesh 2,25 mm. ; 

 width 1,23 mm. 



The solid knot of the spicules and the form and arrange- 

 ment of the mesh correspond with the structure of a spe- 

 cies of Leptopliragma, from the Upper Chalk of Vordorf in 

 Brunswick, but the diameter of the mesh in the Horstead 

 examples is nearl}- twice that of the Vordorf sponge. The 

 hexradiate form of the .spicules is frequently obscured by the 

 attachment of some of the .spicules in an irregular manner to 



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