58 MANUAL OF ZOOLOGY. 



singularly close resemblance to the fauna of the Cretaceous 

 epoch. 



DISTRIBUTION OF SPONGES IN TIME. Remains of sponges 

 are known to occur in formations belonging to the Palaeozoic, 

 Mesozoic, and K.ainozoic epochs. The keratose or horny 

 sponges are obviously incapable of leaving any evidence of 

 their existence, otherwise than by the preservation of the 

 spicula with which the skeleton is furnished ; and such are 

 occasionally found, though they are of rare occurrence. The 

 calcareous sponges are found from the Silurian rocks upwards, 

 attaining their maximum in the seas of the Secondary epoch, 

 the Chalk being especially characterised by their presence. 

 The most important group of fossil sponges is that known as 

 the PetrospongiadcB, characterised by the possession of a 

 stony reticulate framework or skeleton, and by the absence 

 of spicula. The most important genera of this group are 

 Sparsispongia (Devonian) and Ventriculites (Chalk). 



Of the Palaeozoic sponges, Archceocyathus is found in the 

 Potsdam sandstone of North America (Upper Cambrian ?) ; 

 Palceospongia and Acanthospongia are familiar Lower Silurian 

 forms ; and Amphispongia and Favospongia occur along with 

 other forms in the Ludlow rocks. In the Devonian rocks 

 sponges occur pretty frequently, Sparsispongia being the 

 commonest genus. (The Devonian Steganodictyum is really 

 the cephalic buckler of a pteraspidean fish.) The most im- 

 portant Mesozoic genera of sponges are Ventriculites and 

 Siplwnia ; and the order appears, upon the whole, to attain its 

 maximum in the Cretaceous epoch. There seems no reason to 

 doubt but that many of the chalk-flints owe their origin to 

 sponges ; and in some sections of flint are found ' minute 

 spherical bodies covered with radiating and multicuspid 

 spines,' which have been termed Spiniferites or Xanthidia, and 

 are probably the 'gemmules' of sponges. (By some, how- 

 ever, these bodies are regarded as being the ' sporangia ' of 

 Desmidice, an order of the Protophyta.) Many Cretaceous and 

 Tertiary shells are found to be mined by a species of boring 

 sponge, which is nearly allied to the recent Cliona. 



AFFINITIES OF SPONGES. As already pointed out, the sponges 

 are allied both to the Amoebce and to the Foraminifera. Indeed 

 the individual ' sarcoids' or sponge-particles can scarcely be 

 distinguished, when detached, from Amoebae. The sponges 

 ehow likewise a decided relationship to the Radiolaria ; and by 

 Professor James Clark, they are believed to be nearly allied to 

 the 'flagellate' Infusoria. 



