420 M. K. A. Zittel on Fossil HexactineUida. 



radiates octahedrally perforated ; arms of the sexradiates with 

 spinous and root-like processes. 



CoBloptycMum^ Goldf. Cretaceous. 



Suborder II. Lyssakina. 



Entire skeleton consisting of spicules which are united only 

 by sarcode (exceptionally also by lamellar siliceous matter in 

 an irregular fashion). Flesh-spicules generally present in 

 abundance and much differentiated. 



Family 1. Monakidse, Marshall. 



Entire sponge-body composed only of homogeneous spicules. 



AcantJiosj)ongia, M'Coy. Silurian, Carboniferous. 

 Stauractinella, Zitt. Jura. 



Family 2. PleionakidsB, Marshall. 



Principal mass of the skeleton formed of pure sexradiates, 

 but with these broom-forks or rosettes. 



^Askonema, Kent. 

 ^Lanuginella, Schmidt. 



Family 3. PollakidaB, Marshall. 



Forms of the skeleton- and flesh-spicules very multifarious. 

 Special dermal skeleton and lining of the gastral cavities pre- 

 sent. Base generally forming a root-tuft of long siliceous 

 spicules. 



*HoUema, Schmidt. 

 ^Pherone7na, Leidy. 

 * Crateromorpha, Gray. 

 ^Bossellay Carter. 

 * Si/mpagella, Schmidt. 

 ^Flacodt'ctyon, Schmidt. 

 ^Euplectella, Owen. 

 '^Habrodictyon^ W. Thomson. 

 '^Laharia, Gray. 



^ Semperella, Marshall [Meyerina^ Gray). 

 '^Hyalonema {Carteria), Gray. 

 ?Acestra, Rom. Silurian. 



Conditions of Existence and Distribution of the Fossil 

 Hexactinellida. 



In his first, frequently cited memoir, W. Marshall gives a 

 tabular revision of the geographical and bathymetric distri- 



