CLASSIFICATION. 87 



occurrence in the fossil state. Detached monactinellid spicules are, however, very 

 abundant in certain strata, but though many of these most probably belong to the 

 next sub-order, there are others which may undoubtedly be regarded as pertaining 

 to this group. The families of the existing Monactinellida3 are, to a certain extent, 

 based on characters which cannot be applied to the fossil spicules ; but the 

 following appear to be represented : Halichondridse, Desmacidonidre, Suberitidee, 

 and Spongillidas. 



Sub-Order 2. — Tetractinbllid^. 



Sponges with skeletal-spicules normally of four rays or axes, disposed like the 

 four axes of a regular three-sided pyramid. One ray frequently more developed 

 than the others, so as to form a long shaft. Uniaxial spicules of large size usually 

 present. Skeletal-spicules frequently disposed in fascicles with a radiate arrange- 

 ment. Spicules held together by spongin. Polyaxonal spicules in the form of 

 stellates, globostellates, and discs frequently present. 



Representatives of two at least of the existing families of this sub-order, Geodidse 

 and Ancorinidse, are present as fossils, but from their imperfect condition of preser- 

 vation it is impracticable to determine in most of the fossil forms the characters 

 which distinguish the respective families. 



Sub-Order 3. — Lithistid2B. 



Sponges of a stony character, usually with thick massive walls. Skeletal- 

 spicules either four-rayed or altogether irregular in form, usually branching at 

 their extremities, which terminate obtusely, or with minute expanded surfaces. 

 Spicules firmly united together by the intertwining of their branches, or by the 

 close apposition of their expanded extremities, but not fused together. A surface 

 or dermal layer of trifid spicules or small discs is usually developed, as well as 

 minute uniaxial flesh-spicules. Large skeletal uniaxial spicules occasionally 

 present. 



This sub-order has been divided by Zittel into the following four families : 



Family 1. — Rhizomorina. 



Skeletal-spicules usually elongate and irregularly branching, with minute pro- 

 jecting spines. Branches terminating in minute facets, which are closely apposed 

 to the axis and branches of adjoining spicules, forming an irregular meshwork or 



