ORDER IV 



SILICISPONGIAE— HEXACTINELLIDA 



63 



According to Hinde, Tholiasterella and Asteradinella constitute a distinct 

 order (Heteradinellidae), and Astraeospongia is made the type of the order 

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

 Hexactinellids, in which sui:)ernumerary rays are produced by branching. 



Suborder 2. DICTYONINA Zittel. 



Skeletal spicules cemented to form a continuous framework in sncli a way that 

 every arm of a 

 hexactin is applied 

 to the corresponding 

 arm of an adjacent 

 spicule, and both 

 rays become en- 

 veloped in a common 

 siliceous covering. 

 Boot-tuft absent. 



The Dictyonina 

 are probably de- 

 scendants of the 

 Lyssacina (possi- 

 bly from Proto- 

 spongia- and Dictyo- 

 phyton-Wke 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 



by cavities. The more important fossil 

 forms are divided into the following 

 families. 



Family 1. Craticularidae Rauff. (Eure- 

 tidae p. p., Zittel non Schulze.) 



Cup-shaped, cylindrical, branching or 

 flattened sponges. Spicular nodes solid. 

 External surface without distinct dermal 

 layer, but protected by a thickening of the 

 outer skeletal layer, and occasionally covered 

 with a delicate tveb of cemented spicules. 

 Canals simple, blindly terminating in the 

 skeleton. Jurassic. 



Fig. 69. 



Tremadictyon retimdatum (Goldf.). Upper Jurassic ; Streitberg, Pranconia. a, 

 Sponge, "-^/g natural size ; h, Enlarged portion of outer surface witliout dermal layer ; 

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



Fio. 70. 



Craiicidaria -paradoxa (Miinster). Upper Jurassic ; 

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

 6, Latticed skeleton, 12/1 ; c, Tliickened dermal layer. 



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 irregular 

 cubical meshes. Very common in Upper Jurassic. 



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



