PORIFERA. 



57 



deposits, the spicules of Calcareous Sponges are found in a condition not 

 recognisably different from that of recent examples. As a rule, " the 

 spicular fibres of fossil Calcisponges retain their original structure of 

 carbonate of lime, though the form of the component spicules has to a 

 large extent disappeared, and the fibres are now either of crystalline 

 grains of calcite or show a finely radiate prismatic structure" (Hinde). 

 In certain cases, however, the calcareous skeleton of a Calcisponge may 

 be found to be replaced, as occurs commonly in the case of Corals and 

 other calcareous organisms, by chalcedonic silica. Such "silicified" 

 specimens have necessarily undergone an entire destruction of the 

 minute structure of the skeleton, though the form of the skeleton may be 

 perfectly well preserved. 



As regards their general geological distribution^ the Sponges are 

 very largely represented in past time, and have a very high an- 

 tiquity. The two recent groups of the Myxospongice and Cerato- 

 spo?igice, being devoid of hard parts or having only a horny skeleton, 

 are unknown in the fossil condition, though doubtful examples of 

 the latter have been brought 

 forward. The Sponges with a 

 skeleton of siliceous spicules are 

 known to occur in deposits as 

 ancient as the Cambrian, the 

 oldest Sponge at present known 

 being the Protospoiigia fenestrata 

 (fig. 50) of the Menevian Slates 

 of St David's, South Wales. In 

 strata of Ordovician age the re- 

 mains of Sponges are not very 

 uncommon, the groups of the 

 Lithistidce and Hexactinellidce be- 

 ing both represented, the former 

 by types like Hindia, and the 

 latter by Hyalostelia, Recepta- 

 culites, and Ischadites. In the 

 Silurian deposits the same groups 

 of Sponges are well represented 



by forms such as Aulocopiiwi, Astylospongia, and Dictyophyton ; 

 while the group of the Monactinellid Sponges is represented by the 

 genus Climacospongia. If we except the aberrant group of the Re- 

 ceptaculitidce, the Devonian system has, so far, proved to be singularly 

 barren of Sponge remains, though a few types (Astrceospo?igia, 

 Dictyophytoji, &c.) have been detected in strata of this age. On 

 the other hand, the seas of the Carboniferous period were tenanted 

 by a vast abundance of Sponges, belonging to few generic types — 

 so far as yet known — but representing the distinct groups of the 

 Monactinellidce, Tetractinellidc?, Lithistidce, Hexactinellidce, and 



Fig. 50. — Part ol the spicular mesh of Proto- 

 sfiongia fenestrata, enlarged five diameters, 

 from the Menevian strata of South Wales. The 

 spicules are slightly distorted by cleavage. 

 (After Hinde.) 



