Chalk Formation at Coesfeld in Westphalia. Good _ illustra- 
tions of the structure are given in Zittel’s «Handbuch der 
Palaeontologie» p. 175. 
Ventriculites? sp. 
@late Vi, figs: 25, 26): 
Skeletal mesh work composed of spicules with stout arms 
and comparatively large open or «lantern» nodes, the inter- 
spaces are nearly circular. Distance from node to node 0,33 mm. 
Thickness of the arms 0,05 mm.; and of the central «lantern» 
node 0,15 mm. Fragments of surface layer with irregularly 
disposed circular pores. 
The skeletal mesh of this form may be readily distinguish- 
ed from the preceding by the stouter arms, the smaller and 
nearly circular interspaces, and their more regular arrangement. 
These differences appear sufficient to show that there are in 
this Horstead material two different species of hexactinellid 
sponge with the open or «lantern» nodes. I am, however, un- 
certain whether these fragments properly belong to the genus 
Ventriculites, for the mesh has a much more regular arrange- 
ment than in any of the specimens of Ventriculites which 
I have seen, and corresponds closer with the mesh of the 
allied Genus Pachyteichisma, Zittel. As however the Genus 
Ventriculites is known to occur in the Norfolk Chalk, I prefer 
to place these fragments provisionally under it. 
Sub-Order Lyssakina, Zittel. 
In this division of the hexactineilid sponges the spicules 
are not welded together by the extremities of the arms, but 
are merely held together by the sarcode of the living animal. 
Consequently in the fossil condition, the spicules are as a 
rule detached from each other, though occasionally the form 
of the sponge is still retained. | 
