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



radiates octaliedrally perforated ; arms of the sexradiates with 

 spinous and root-like processes. 



Coeloptychiumj 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. MonakidaB, Marshall. 



Entire sponge-body composed only of homogeneous spicules. 



Acantlwspongia^ M'Coy. Silurian, Carboniferous. 

 Stauractinellaj Zitt. Jura. 



Family 2. Pleionakidse, Marshall. 



Principal mass of the skeleton formed of pure sexradiates, 

 but with these broom-forks or rosettes. 



^ Askonema, Kent. 

 ^Lanuginella, Schmidt. 



Family 3. PoUakidae, 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. 



'^Holtenia, Schmidt. 



'^Pheronema^ Leidy. 



* Crateromorpha^ Gray. 



^RosseUa, Carter. 



* SyvipageJla^ Schmidt. 



*Placodictyo7i^ Schmidt. 



'^Euplectella^ Owen. 



^Hahrodictyon, W. Thomson. 



'^Lahariaj Gray. , 



"^ Semper ella^ Marshall [Meyerina, Gray). 



'^Hyalonema [Carteria], Gray. 



?Acestraf Rom. Silurian. 



Conditions of Existence and Disfrihution of the Fossil 

 Hexactin ellida. 



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

 tabular revision of the geographical and bathymetric distri- 



