REPORT ON THE HEXACTINELLIDA. 13 



Family 4. Mellitionid^, Zittel. 



Sponge body branched, spherical or flat. Body-wall completely perforated by nume- 

 rous tubular water canals, and thus divided into honeycomb-like cells. Skeletal spicules 

 with thick nodes of intersection. Surface (naked or) covered by a delicate, meshed or 

 porous, siliceous skin, which also covers the openings of the canals. Root tuft absent. 



Examples. — The genus Aphrocallistes, Gray, which is known both in the living and 

 fossil form, the living genus (X) Fieldingia, Saville Kent, and the fossil genus Stauronema, 

 Sollas. 



Family 5. Ventriculiti d^, Zittel. 



Sponge body simple or polyzoic, beaker-, funnel-, or top-like, cylindrical or branched. 

 Wall irregularly folded. Lattice framework with octahedral perforated nodes of inter- 

 section. Canal system usually well developed. Radial canals blind. Both surfaces 

 with ostia or longitudinal furrows. Dermal layer seldom absent, and usually formed by 

 thickening of the outer skeletal layer. Roots consisting of prolonged siliceous fibres 

 united by transverse bridges and without axial canals. 



Examples. — Ventriculites and other fossil genera. 



Family 6. Staurodermi dm, Zittel. 



Sponge body top- or funnel-like, seldom branched. Lattice skeleton more or less 

 reo'ular. Intersection nodes thick or octahecbal, perforated. The outer, or both surfaces 

 of the wall provided with stellate spicules, which difi"er in form from those of the rest of 

 the skeleton, and are either but loosely cemented to one another, or lie embedded in a 

 continuous siliceous skin. 



Examples. — Stam-oderma and other fossil genera. 



Family 7. M^andro3pongid.e, Zittel. 



Sponge body consisting of intricately labyrinthine and anastomosing thin-walled tubes 

 or foUse. Canal system absent or scarcely developed. Intercanalicular system always 

 present. Dermal layer absent or represented by a continuous siliceous superficial skin. 



Examples. — Besides Cystispongia and other fossil genera, the living genera Dactylo- 

 calyx, Stuchbury, Periphragella, Marshall, and Myliusia, Gray {pro parte). 



Family 8. Callodictyonid.e, Zittel. 



Sponge body beaker-llke. Wall consisting of a regular wide-meshed lattice-like 

 framework with octahedral intersection nodes ; canal system absent or confined to the 

 sometimes very thick dermal layer of the lattice-like skeleton. 



Examples. — Callodictyon, Zittel, and other fossil genera. 



