PORIFERA. DEMOSPONGI.E 43 



canals opening into its base. "Wall formed of anastomosing fibres, 

 between which are irregular spaces — there are no distinct canals; 

 fibres formed of large spicules, branched and interlaced. There is 

 also a surface layer composed of small spicules. Chalk. Ex. P. 

 scriptum, Upper Chalk. 



Scytalia. Simple, or formed of two or more individuals 

 growing close together ; cylindrical or club-shaped, with a thick 

 wall and a cylindrical stem. Central cavity tube-like, long, con- 

 tinued at its base by several vertical canals ; numerous radial 

 canals open into the central cavity and taper toward the external 

 surface. Spicules branching, with root-like prolongations. Chalk. 

 Ex. S. radiciformis. 



Seliscothon. Mushroom-like, consisting of a flat or concave, 

 circular, plate-like body, and a rounded tapering stem. The circular 

 body has rounded or oblique edges, and numerous, small, rounded 

 oscula on the upper surface ; it is formed of fine vertical radiating 

 lamella?, separated by spaces crossed by fibres — these spaces forming 

 the canal-system. Spicules fine, branching irregularly, with bifur- 

 cating extremities, and covered with tubercles or spines. Chalk. 

 Ex. S. planus. 



Doryderma. Cylindrical, pear-shaped, sometimes branching. 

 There are parallel vertical canals opening at the summit of the 

 sponge, and smaller radial canals extending from the surface towards 

 the centre. Spicules large, of various forms ; also a surface layer 

 formed of slender trifid spicules. Carboniferous and Cretaceous. 

 Ex. D. benetti, Upper Greensand. 



Siphonia. (fig. 11.) Pear-shaped, usually provided with a 

 stalk, which is given off from the broad end of the body and termi- 

 nates in rootlets. The incurrent canals are small, slightly curved, 

 and extend radially from the centre of the sponge to the surface. The 

 excurrent canals are larger, and are arranged parallel with the surface 

 of the sponge, extending from the base to the summit, where they 

 open into the large central cavity by means of a series of parallel 

 ostia. The skeletal-spicules possess four rays with bifurcated and 

 expanded extremities, by means of which they are interlocked. 

 Upper Greensand to Upper Chalk. Ex. S. tulipa, Upper Green- 

 sand. 



