88 BRITISH FOSSIL SPONGES. 



confused fibres. A canal in the main axis of the spicules. Dermal layer either of 

 spicules resembling those of the skeleton, or of compound trifids. 



Family 2. — Megamorina. 



Skeletal-spicules large, elongate, smooth, straight or curved, simple, or irregu- 

 larly branching. A canal in the main axis of the spicule. Branches terminating 

 obtusely or with expanded facets. Spicules united by being intertwined together 

 or by the apposition of their terminal facets, forming an open meshwork. Small 

 spicules of the Rhizomorina type occasionally filling the interspaces of the mesh. 

 Dermal layer of compound trifid, or minute uniaxial spicules. 



Family 3. — Anomocladina. 



Skeletal-spicules consisting of a central sub-spherical node, from which a variable 

 number of simple or furcate arms, with slightly expanded terminations, radiate 

 in different directions. In some cases the nodes are in twins connected by a short 

 axis, in which a canal is present, and the rays are partially spined. Spicules united 

 by the apposition of the expanded terminations of the rays to the nodes, and 

 occasionally to the branches, of adjoining spicules, so as to form a regular mesh- 

 work. 



Family 4. — Tetracladina. 



Skeletal-spicules of four rays extending from a common, non-inflated centre, 

 approximately at angles of 120°. Rays smooth or tuberculated, with branching 

 extremities, which interlock with those of adjoining spicules to form the mesh. 

 An axial canal in each of the spicular rays. The spicules of the dermal layer are 

 either compound trifids, entire or lobate discs, or irregular iaminaB, and also 

 minute uniaxial spicules. 



Sub-Order 4. — Hexactinellid^e. 



The skeletal-spicules normally consist of six rays, radiating from a common 

 centre, at right angles to each other. An axial canal is present in each ray. The 

 spicules are arranged so that their rays overlap each other and form a lattice-like 

 mesh with cubical or irregular interspaces. They are either united together by 

 a common siliceous envelope, or interlaced and held in position by the soft struc- 

 tures. Two main groups Dictyonina, Zitt., and Lyssakina, Zitt. 



