46 



COELEN TER AT A SPONGI AE 



CLASS I 



tributed 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 soldered together, the latter become detached and strewn in all 

 directions. While Monactinellid spicules are very common in certain forma- 

 tions, they are rarely united in the form of coherent skeletons, and are only 

 capable of generic determination when possessing characteristic forms (Remeria, 

 Esperia, etc.) The lowermost members of the Alpine Lias often contain 

 considerable hornstone, and are sometimes completely filled with rod-shaped 

 spicules. In various Cretaceous and Tertiary horizons also Monactinellid 

 spicules are enormously abundant. Hinde has described a Climacospongia 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 silicious elements which are also encased in horny fibres, 

 and by means of which they bore labyrinthic passages in the shells of mollusks. 

 Fossil sponge -borings are also common. Detached spicules of Renieria, 

 Axinella, and Haplistion have been described by Hinde from the English Car- 

 boniferous Limestone. 



Order 2. TETRACTINELLIDA. Marshall. 

 (Tetraxonia, F. E. Schulze.) 



Skeleton composed of regular tetraxons which are generally combined with uni- 

 axial, polyaxile, or heteraxile silicious bodies. The skeletal elements occur detached 



throughout the soft parts, and are never united to 

 form a connected framework. 



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 

 fascicles, and are surrounded by a thick layer of 

 anaxile spheres. 



Detached Tetractinellid spicules associated 

 with Monactinellids occur more or less abundantly 

 in the Carboniferous Limestone, the Alpine Infra- 

 Lias, the English Neocomian, 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 

 Tethyopsis, Zittel (Fig. 52); and PacJiastrella, 



FlG - 52 - 



genera Ophiraphidites, Carter 

 Schmidt. 



Order 3. LITHISTIDA. Schmidt. 



Massive, thick -walled, silicious sponges, usually with complicated canal -system. 

 Skeleton composed of irregular tetraxons or monaxons (desmoms), which develop knotty 

 or root-like branches either at the extremities or all along the shaft, and are firmly 



