92 BRITISH FOSSIL SPONGES. 



are frequently reduced or absent, form a vertical axis. Spicules merely held in 

 position by the soft structures of the Sponge. This sub-order is represented by the 

 single genus Astrceospongia, Ferd. Roemer. 



Sub-Order 6. — Heteractinellidje. 



Skeletal-spicules consisting of an indefinite number of rays, varying from six 

 to thirty, radiating from a common centre. The body-spicules irregularly 

 arranged and held in position by the soft structures of the Sponge. The spicules 

 of the dermal layer are interwoven together, and their rays partially or completely 

 fused with each other. 



Order IV. — Calcispongi^. 



Sponges whose skeletal spicules are composed of carbonate of lime. Spicules 

 either uniaxial, three-rayed, or four-rayed. They are either regularly arranged to 

 form the skeleton, or loosely distributed in the soft tissues, or closely apposed 

 together to form anastomosing fibres. Only two families have up to the present 

 been definitely recognised in the fossil state. 



Family, Pharetrones, Zitt. 



Skeletal-spicules arranged in the form of solid anastomosing fibres. Canals 

 branching irregularly, at times not indicated in the skeleton. Dermal layer 

 forming a continuous smooth or corrugated membrane. 



Family, Stcones, Haechel. 



Skeletal-spicules very regularly arranged to form transverse simple radial 

 tubes or chambers, which open into a central cloaca. A distinct dermal and 

 cloacal layer present. 



