46 COELENTERATA—SPONGIAE CLASS I 
tributed throughout the body. Needles, hooks, crotchets, cylinders, spindles, 
amphidises, and the like oceur 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 (Ltenieria, 
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 trom 
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 Lenieria, 
Avinella, and Haplistion have been described by Hinde from the English Car- 
boniferous Limestone. 
Order 2, TETRACTINELLIDA. Marshall. 
(Tetraxonia, F. KE. 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 (Hails), the Upper Cretaceous of Haldem and 
Coesfeld in Westphalia, and in the Tertiary and 
Pe A SL Sa Pleistocene formations. The skeletal elements 
are preserved in their natural position in the 
genera Ophiraphidites, Carter ; Tethyopsis, Zittel (Fig. 52); and Pachastrella, 
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 
