— 65 — 
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 
5 
