prinxipal groups of the sponges. 167 



In the Ordovician rocks we meet with species of the genera 

 Hindia and Astylospongia, both of which are represented, still more 

 abundantly in Silurian strata. The genus Astylospongia (fig. 57) 

 comprises globular or ovate, unattached sponges, furnished with a 

 cup-shaped or funnel-shaped depression at the summit. There are 

 two principal sets of aquiferous canals, those of the one series being 

 radial and directed from the surface towards the centre, while those 

 of the other series are radial, but are vertically disposed in a direction 

 parallel to the surface, so as ultimately to open into the summit-cup. 

 Astylospongia was formerly referred to the Hexactinellids, but recent 

 researches have shown that it is truly referable to the Anomocladine 

 group of the Lithistids. The skeleton consists of siliceous spicules 

 (fig. 55, a) with solid nodes, from which radiate from six to nine 

 straight rays, the ends of which are branched and are furnished with 

 slightly expanded articular processes. The rigid skeletal network is 

 formed by the apposition of the branched ends of the spicules to the 

 nodes of adjoining spicules, or, sometimes, to the ends of their 

 immediate neighbours. More or less closely allied to Astylospongia 

 are the Silurian genera Palceomanon and Protachilleum, and probably 

 the Ordovician genus Eospongia ; while the Jurassic genera Melonella 

 and Cylindrophyma, the Cretaceous Mastosia, and the recent Vetulina 

 are regarded by Zittel as belonging to the same family. 



The genus Hindia comprises free, spherical or subspherical 

 sponges, in which the body is traversed by numerous canals which 

 radiate from a central space and open on the surface. The skeleton 

 consists of four-rayed spicules, the digitated ends of which embrace 

 the nodes or rays of adjoining spicules in such a way as to form a 

 regular meshwork. The species of the genus are chiefly found in 

 the Ordovician and Silurian rocks, but spicules have been recognised 

 by Hinde in strata of Carboniferous age. 



No Devonian Lithistids are at present known, but a few forms of 

 this group {Cnemidiastrum and Dory derma) have been recognised 

 in the Carboniferous deposits. No forms of the Lithistidce nre 

 known in the Permian or Triassic rocks, but very numerous types 

 of this order of sponges have been recognised as occurring in the 

 Jurassic deposits. In the Upper Jurassic rocks of Germany, in 

 particular, are numerous Lithistids, belonging to such genera as 

 Cnemidiastrum, Hyalotragos, Platychonia, Placojiella, <xx. It is, 

 however, in the upper portion of the Cretaceous system that the 

 Lithistid Sponges attain their maximum development, the genera 

 Chenendopora, Siplionia, Vei-ruculina, Chonella, Jei-ea, Jereica, Bol- 

 idium, Turonia, Plinthosella, &c, being characteristic and widely 

 distributed types. Of all the Cretaceous Lithistids none is more 

 generally familiar than the genus Siplionia (fig. 58), in which the 

 sponge consists of a pyriform or subspherical body, supported 



