- 6 5 - 



An approximation to. the affinities of these fragments of the 

 skeleton may be made by a comparison of the regular or 

 irregular disposition 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 by the surface membrane, and the connection be- 

 tween this outer membrane and the interior skeleton is occa- 

 sionally shown by fragments in which a layer of the interior 

 skeleton spicules yet remains attached to the surface membrane. 

 Unfortunately most of these hexactinellid fragments have been 

 injured by the fossilization ; the interior canals have been 

 obliterated, the small processes forming the lantern knots have 

 been worn through, and the spicular arms have been so eroded 

 that it is difficult to tell definitely whether they were origi 

 nally smooth or spinous. 



Sub-order Dictyonina, Zittel. 



Genus Leptophragma, Zittel 1877. 



Leptophragma sp. 



(Plate V, fig. 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 slightly 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 Leptophragma, from the Upper Chalk of Vordorf in 

 Brunswick, but the diameter of the mesh in the Horstead 

 examples is nearly 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 



