THE MOST ANCIENT SPONGES. 477 



Orders and Classes of which it is composed, and weighing the influ- 

 ence of these elements of faunas upon each other, in the light of such 

 knowledge as is available of the zoological functions of similar groups 

 of organisms in existing natural history provinces or regions. 



We now present a brief abstract of the succession of life on the 

 earth in geological time, with a view to direct attention to some of the 

 principal elements in fossil groups of life. No attempt at numerical 

 estimates of individuals or species is made; and attention is only 

 drawn to some of the commoner generic types in the more abundant 

 Orders and Classes in which the palaeontologist may study the main 

 facts of the succession of life. 



Many of the genera mentioned are only to be found in great public 

 collections like the museums of Universities, Colleges, and the British 

 Museum ; which are to the palaeontologist what a lexicon is to the 

 student of a language, and must be used for reference. In any case 

 it is to be desired that, before endeavouring to grasp the grand pro- 

 blems of distribution, evolution, and succession of life which the strata 

 unfold through their physical geology and their fossils, some such 

 elementary study should be gone through as is implied in the practical 

 examination of the generic types in the chief natural history groups 

 which we now enumerate. 



Spongia. Sponges which occur fossil are necessarily siliceous or 

 calcareous. The siliceous sponges are chiefly hexactinellid, having 

 six-rayed spiculae, or lithistid, having four-rayed spiculae or an irregular 

 spicular structure. The spiculse are usually united together, and in 

 the lithistid sponge the mesh is generally dense. Siliceous spiculae 

 may be replaced by calcite, peroxide of iron, and iron pyrites. Sponges 

 are chiefly known from sandstones and limestones. They are com- 

 paratively rare in the Primary rocks, from which calcareous sponges 

 are unknown, if we except Peronella, which appears in the Devonian, 

 but is otherwise found only in the Secondary strata. The Monacti- 

 nellid sponges are represented in the Primary rocks, though the fossil 

 genera are few. 



The oldest known sponge is Protospongia ; it belongs to the Hexac- 

 tinellid group and is found in the Menevian rocks. Other primary 

 hexactinellid sponges are species of Astylospongia, Palceomanon, Bra- 

 chiospongia, Dictyophyton, Plectoderma, Protachilleum, and the Lyssa- 

 kine Hexactinellid genera Astriovpongia, Hyalostelia, Holasterella, arid 

 Amphispongia. The Tetractinellids are represented in the Carbonifer- 

 ous rocks by the species of Geodia and Pachastrella. The Lithistid 

 sponges of the Primary strata comprise species of Doryderma, Hindia, 

 and Aulocopium. The MonactineHid sponges include Climacospongia, 

 Lasiocladia, Haplistion. 



With the Secondary rocks the Monactinellid sponges become more 

 numerous ; the living genus Spongilla occurs in the Purbeck limestone, 

 but most of the types of this group are Cretaceous, among which may 

 be mentioned Dirrhopalicm, Acanthomphis, and Cliona ; the latter 

 ranging through the Tertiary deposits. The Tetractinellid sponges 

 are chiefly Cretaceous ; and among the genera which appear in the 



