60 COELENTERATA— PORIFERA phylum ii 



in which the externally directed ray has become atrophied ; or a dense siliceous 

 envelope is secreted, in which stellate hexactins with reduced outwardly 

 and inwardly directed rays (stauractins) are embedded in greater or lesser 

 profusion. 



The Hexactinellida of the present day are distributed chiefly over the greater 

 depths of the ocean beyond the hundred-fathom line (200 to 3000 fathoms). 

 They occur fossil principally in deep-sea deposits, and make their first appear- 

 ance in the Cambrian ; their period of greatest development coincides with 

 Jurassic and Cretaceous time. 



Suborder 1. LYSSACINA Zittel. ^ 



Skeletal elements either entirely detached, or only partially and in an irregulär 

 fashion cemented together. Root-tufl offen present. 



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-sjpicules are invariably destroyed. Notwithstanding, 

 complete sponges composed of large-sized detached hexactins have been found 

 in Paleozoic 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, sack-, tuhe-like or spherical sponges, with walls composed of a Single 

 layer of cruciform tetraxial spicules (stauractins), arranged so as to form quadrate 

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

 larger elements is divided into secondary Squares hy 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 Fhormosella Hinde. 



Family 2. Dictyospongidae Hall. 



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

 are frequently diver sified by ridges and prominences. Skeletal framework very regulär, 

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

 Framework formed by bundles of slender spicules. Chiefly in Devonian (Chemung) 

 and Lower Carboniferous (Keokuk) of North America, and Devonian of Europe. 



Subfamily 1. Dictyospongiinae Hall and Clarke. 



Hall and Clarke. Smooth, obconical or subcylindrical 

 sponges devoid of nodes, ridges or other ornamentation ; base furnished with 

 a tuft of long, straight, anchoring spicules. Silurian and Devonian. 



Hydnoceras Conrad (Fig. 64). Obconical more or less rapidly expanding 

 sponges with eight prism-faces and nodes in horizontal and vertical rows. Base 

 with short tuft of anchoring spicules. Devonian and Carboniferous. 



Lysactinella Grirty ; Hydriodictya, Prismodictya, Gongylospongia, Botryodictya, 

 Helicodictya, Rhabdosispongia, Ceratodictya, Clathrospongia, Lebedicfya Hall and 

 Clarke. Chemung Group ; New York. 



