6g- - 



Chalk Formation at Coesfeld in Westphalia. Good illustra- 

 tions of the structure are given in Zittel's Handbuch der 

 Palaeontologies p. 175. 



Ventriculites? sp. 



(Plate V, 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 hexactinellid 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. 



