ORDER II 



SILICISPONGIAE— TETRACTINELLIDA 



51 



(Spongilla) that are known. The skeleton, as a rule, is composed like that 

 of the horny sponges, of anastomosing spongin fibres, which either encase 

 rod-like spicules, or contain quantities of uniaxial siliceous elements ; some- 

 times the latter are also present in the soft parts. In each genus there are 

 usually either one or but few varieties of siliceous elements present, which are uni- 

 formly distributed throughout the body. Needles, hooks, crotchets, cylinders, 

 spindles, amphidiscs and the like occur in great diversity. Owing to the 

 decomposition of the horny fibres during fossilisation, and the fact that the 

 skeletal elements are never fused together, the latter become detached and 

 scattered in all directions. While Monactinellid spicules are very common 

 in certain formations, they are rarely united in the form of coherent 

 skeletons, and are only capable of generic determination when their form is 

 sufficiently characteristic, as in Benieria, Esperia, etc. The basal beds of the 

 Alpine Lias often contain considerable hornstone, which is sometimes com- 

 pletely filled with rod-shaped spicules. In various Cretaceous and Tertiary 

 horizons Monactinellid spicules are also enormously abundant. Hinde has 

 described a Climacosjwngia from the Silurian of Tennessee, in which the 

 skeleton consists of spicules arranged in longitudinal rows, and connected by 

 transversely disposed elements. The spicules were probably originally 

 enclosed in horny fibres. The Clionidae secrete pin-shaped siliceous elements 

 which are also encased in horny fibres, and Eecent sponges of this family 

 bore labyrinthic passages in the shells of mollusks. Fossil sponge-borings are 

 also common. Detached spicules of Benieria, Axinella and HajMstion have 

 been described by Hinde from the English Carboniferous Limestone. 



Order 2. TETRACTINELLIDA Marshall. 



{Tetraxonia F. E. Schulze.) 



Skeleton composed of regidar fetraxons which are generally combined loith uni- 

 axial, polyaxial or heteraxial siliceous bodies. The 

 skeletal elements occur detached throughout the soft 

 parts, and are never united to form a connected frame- 

 work. 



The most common forms of skeletal elements 

 are normal tetraxons, anchors with simple or 

 furcate prongs, spheres and stellate bodies. In 

 certain genera (Geodia) the large anchors and 

 cylinders are disposed in radiately arranged fasci- 

 cles, and are surrounded by a thick layer of 

 anaxial spheres. 



Detached Tetractinellid spicules associated 

 with Monactinellids occur more or less abundantly 

 in the Carboniferous Limestone, the Alpine Infra- 

 Lias, the English Neocomain, the Deister Sand- 

 stone (Hils), the Upper Cretaceous of Haldem 

 and Coesfeld in Westphalia, and in the Tertiary 

 and Pleistocene formations. The skeletal elements are preserved in their 

 natural position in the genera Ophiraphidifes Carter ; Tethyopsis Zittel (Fig. 47), 

 Pachastrella Schmidt, Stolleya and Cephaloraphidites Schrammen. 



Fig. 47. 



Tethyopsis steinmanni Zittel. Upper- 

 Cretaceous ; Ahlten, Hanover, i^/i. 



