ORDER IV 



SILICISPONGIAE— HEXACTINELLIDA 



63 



According to Hiiide, Tholiasterella and AsteracMnella constitute a distinct 

 Order (Heteraäinellidae), and AstraeosjJongia is made the type of the order 

 Octadinellidae. These two groups may perhaps best be regarded as aberrant 

 Hexactinellids, in which supernumerary rays are produced by branching. 



Suborder 2. DICTYONINA Zittel. 



Skeletal spicules cemented to form, a continuous framework in such a way that 

 every arm of a „ 



hexaäin is applied 

 to the correspondiiig 

 arm of an adjacent 

 spicule, and hoth 

 rays hecome en- 

 veloped in a common 

 silieeous covering. 

 Root-tuft absent. 



The Dictyonina 

 are probably de- 

 scendants of the 

 Lyssacina (possi- 

 bly from Froto- 

 spongia- and Dictyo- 

 phyton-Mke forms). 

 They appear first 

 in the Trias, and 

 play a prominent 



role as rock-builders in the Jurassic and Cretaceous. Their lattice-like skeletons 



are frequently replaced by calcite, or are dissolved away and merely indicated 



a ' . h by cavities. The more important fossil 



forms are divided into the foUowing 

 families. 



Family 1. Oraticularidae Raiiff. {Eure- 

 tidae p. p., Zittel non Schulze.) 



Cup-shaped, cylindrical, hranching or 

 flatternd sponges. Spicular nodes solid. 

 External surface without distinct dermal 

 layer, hut protected by a thickening of the 

 outer skeletal layer, and occasionally covered 

 with a delicate web of cemented spicules. 

 Canals simple, blindly terminating in the 

 skeleton. Jurassic. 



Tremadictyon Zitt. (Fig. 69). Cup-, 

 plate-shaped or cylindrical, with wide 

 cloaca. Canal-openings on both sides in alternating rows. Base nodular; 

 exterior veiled over with delicate net-work of amalgamated hexactins, extend- 

 ing even across canal pores. Skeletal framework with more or less irregulär 

 cubical meshes. Very common in Upper Jurassic. 



Craticularia Zitt. (Fig. 70). Funnel-shaped, cylindrical or flattened ; 



Fig. 69. 



Tremadictyon reticulatum (Goldf.). Upper Jurassic ; ötreitberg, Franconia. a, 

 Sponge, 2/5 natural size ; b, Enlarged portion of outer surface without dermal layer ; 

 c, Portion with well-preserved dermal layer, 3/j ; d, Skeleton, 12/^. 



Fig. 70. 



Upper Jurassic ; 



Craticularia jmradoxa (Münster). 

 Muggendorf, Franconia. a, Sponge, 1/3 natural size ; 

 b, Latticed skeleton, 12/1 ; c, Thickened dermal layer, 



