ORDER IV SILICISPONGIAE—HEXACTINELLIDA 55 
The Lyssacina 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-sized, detached hexactins, have been found 
in Palaeozoic formations, and also in the Upper Jurassic of Streitberg ; and, 
indeed, the oldest sponges that can be determined with certainty all belong to 
the Lyssacina. ; 
Family 1. Protospongidae. Hinde. 
Thin-walled, sac, tube-like, or spherical sponges, with walls composed of a single 
layer of cruciform tetraxial spicules (stauractins), so arranged as to form quadrate 
and subquadrate meshes. Elements non-fasciculate. The reticulation formed by the 
larger elements is 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 Phormosella, Hinde. 
Family 2. Dictyospongidae. Hall. 
Usually large, funnel-shaped, cylindrical, or prismatic sponges, whose 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 formed by bundles of slender spicules. Ordovician to Devonian. 
Chiefly in Devonian of North America and Europe. 
Dictyophyton, 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. Raut. 
Thin-walled tubes with skeleton composed of a regular framework made wp 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, Waleott ; Palaeosaccus, Acanthodictya, Hinde; Ordovician.  Plec- 
toderma, Hinde ; Silurian. 
Genera incertae sedis. 
Pattersonia, 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. 
Pyritonema, M‘Coy (Acestra, Roem.) Fascicles of long, stout spicules, sup- 
posed to be root-tufts. Silurian. 
Hyalostelia, 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. 
Holasterella, Carter, Spiractinella (Fig. 69), and Acanthactinella, Hinde, are 
allied genera occurring in the Carboniferous Limestone of Great Britain. 
