ORDER iv SILICISPONGIAE HEXACTINELLIDA 55 



The Lif.^iriim are poorly adapted for preservation in the fossil state, since 

 the skeletal elements are but rarely cemented together to form a connected 

 framework, and the flesh-spicules are invariably destroyed. Notwithstanding, 

 complete sponges composed of large-si/ed, detached hexactins, have been found 

 in \ 'alaeozoic formations, and also in the Upper Jurassic of Streitberg ; and. 

 indeed, the oldest sponges that can lie determined with certainty all belong to 

 the Lyssacina. 



Family 1. Protospongidae. Hinde. 





l, >///, fitlit'-like, or spherical sponges, with walls composed of a single 

 In iti- r of cruciform f>'fr<i.dal spicules (stauractins), so arranged as to form quadrate 

 and subquadrate meshes. Elements non-fasciculate. The reticulation formed by the 

 larger fl>-nti'nt* /* divided into secondary squares by smaller spicules, so that the mesh- 

 work is constituted of several series of squares. Cambrian and Ordovician. 



To this family belong the genera Protospongia, Salter, and PhormoseUa, Hinde. 



Family 2. Dictyospongldae. Hall. 



Usually large, funnel-shaped, cylindrical, or prismatic sponges, wlwse thin walls 

 are frequently diversified by ridges and prominences. Skeletal framework very regular, 

 and composed of larger and smaller quadrate meshes situated one within the other. 

 Framework farmed by bundles of slender spicules. Ordovician to Devonian. 

 Chiefly in Devonian of North America and Europe. 



Dictijopliyton, Uphantaenia, Hall, and Hydnoceras, Conrad, occur usually in the 

 form of well-preserved moulds in Devonian sandstones and slate ; the silicious 

 spicules are totally destroyed. 



Family 3. Plectospongidae. Rauff. 



Thin-walled tubes with skeleton composed of a regular framework made up of an 

 ascending and approximately ring-like series of spicules; the latter form rectangular 

 to quadrate, but not very symmetrical meshes. Spicular rays fasciculate. Ordovician 

 and Silurian. 



Cyathophycus, Walcott; Palaeosaccus, Acanthodictya, Hinde; Ordovician. Plec- 

 todenna, Hinde ; Silurian. 



Genera incertae sedis. 



Patter* i a i, Miller (Strobilospongia, Beecher). In form of large botryoidal 

 clumps. Brachiospongia, Marsh. Vase-like sponges with broad inferior margin 

 prolonged into a number of hollow arms; Ordovician of North America. 

 These, together with Amphispongia, Salter, and Astroconia, Sollas, from the 

 Silurian of England, represent extinct families of the Lyssacina. 



I'f/rituneina, M'Coy (Acestra, Koem.) Fascicles of long, stout spicules, sup- 

 posed to be root-tufts. Silurian. 



llii<il.<ti'1i., Zitt. (Acanthospongia, Young). Skeletal elements relatively large, 

 in the form of regular hexactins and stellate bodies with reduced vertical ray, 

 and with inflated nodes. Root-tuft composed of elongated, slightly bent fibres, 

 sometimes terminating in four recurved rays. Cambrian to Lower Carbon- 

 iferous ; Great Britain. 



Il'ilasterella, Carter, Spiradinella (Fig. 69), and Acanthactinella, Hinde, are 

 allied genera occurring in the Carboniferous Limestone of Great Britain. 



