PRINCIPAL GROUPS OF THE SPONGES. 1 63 



tinellid Sponges are almost unknown, but strata of this age in Bel- 

 gium have yielded remains on which the genus Lasiodadia has been 

 founded. In the Carboniferous system, on the other hand, particu- 

 larly in some of the chert-beds associated with the Mountain Lime- 

 stone and Yoredale series, the remains of Monactinellid Sponges are 

 comparatively abundant. The best known types of this age are 

 Reniera and Axinella, the former represented by cylindrical spicules 

 (fig. 52, d and e), and the latter by pin-shaped spicules (fig. 52, f). 

 In the genus Haplistion more or less complete specimens have been 

 found, showing that the sponge possessed a fibrous skeleton com- 

 posed of minute, straight, or curved, acerate spicules. 



In the Permian rocks no certain remains of Monactinellid Sponges 

 are known, the Spongillopsis of Geinitz being apparently inorganic ; 

 and the Trias has also yielded so far no traces of this group. In 

 the Jurassic system also, vast as is the number of known Sponges 

 belonging to other orders, no type of the Monactinellids has hitherto 

 been recognised till the very summit of the formation is reached. 

 In the Purbeck beds, however, in chert containing the remains of the 

 fresh-water C/iara, are found numerous spicules belonging to a fresh- 

 water Sponge, to which the name of Spongilla Purbeckensis has been 

 given (fig. 52, c). In the Cretaceous system, likewise, the remains 

 of Monactinellids are very rare, but the Upper Chalk has yielded 

 shells bored by species of Cliona, and also the skeleton of the genus 

 Acanthoraphis (fig. 52, b), in which the spicules are fusiform and 

 minutely tuberculate. In the Tertiary rocks, lastly, the Monactinel- 

 lids are mostly, if not wholly, represented by the tunnels bored in 

 shells by species of Cliona. 



Order 4. Tetractinellid,e. — This order comprises marine 

 Sponges in which the skeleton consists of siliceous skeleton-spicules, 

 frequently arranged in bundles in a radiating manner, and held to- 

 gether by spongin. The characteristic skeleton-spicules are tetraxial, 

 the commonest form being that of an elongated rod carrying three 

 shorter summit-rays at the upper end (fig. 49, d). In addition to 

 the typical tetraxial spicules there are usually other uniaxial forms, as 

 well as smaller " flesh-spicules," generally of a globate, stellate, or 

 reniform shape. 



The Tetractinellid Sponges are largely represented at the present 

 day, the genera Tethya and Ge&dia being well-known examples of 

 the group. Owing to the want of a continuous siliceous skeleton, 

 however, the Tetractinellids are not commonly preserved as fossils, 

 and they are usually represented by detached spicules only. The 

 oldest known examples of this order occur in the Carboniferous 

 rocks, which have yielded the remains upon which the genera 

 Geodites and Pachastrella have been founded. The former of these 

 is supposed to be allied to the recent Geod/a, and is characterised 



