9 ' INTKODUCTION. 



3. MONACTINELLID^, Zittel. 



Spono'es with skeletons of horny fibres with cores of uniaxial siliceous spicules, or 

 built up wholly of uniaxial siliceous spicules. 



4. Tetractinellid^, Marshall. 

 Spono-es with skeletons of siliceous spicules, usually with four rays or arms, one 

 generally elono-ated to .form a shaft, the other three disposed in pyramidal form ; 

 uniaxial and star-shaped spicules are also present. 



5. LiTHiSTiD^, O. Schmidt. 

 Sponges with skeletons of siliceous spicules, either four-rayed or irregular in form, 

 which are intimately interwoven together into a continuous mesh. 



6. Hexactinellid^, O. Schmidt. 



Sponges with skeletons of six-rayed siliceous spicules, either loosely interwoven 

 together, or organically united to form a continuous mesh. 



7. CalcispongijE, Blainville. 

 Sponges with skeletons of calcareous spicules. 



The first of these existing orders, the Myxospongiae, is, of course, quite unknown 

 in the fossil state, and it is also very doubtful whether any remains of the next 

 order, the Ceratospongiae, have been preserved. Certain casts of cylindrical bodies 

 from the Cretaceous system have been regarded as belonging to horny sponges ; but 

 in the absence of all other characters but that of outward form these bodies cannot be 

 definitely placed in this group. 



The Monactinellidse have comparatively few representatives in the fossil state, and 

 they present a striking contrast to the abundance of this order in the present seas. 

 Their rarity as fossils, however, is not to be accepted as an indication of their scanty 

 existence in the past, but is more probably owing to the fact that the spicular 

 structure of these sponges is unsuitable to their preservation as fossils. 



The structure of the Tetractinellidse, like that of the order just mentioned, is also 

 but little adapted to the retention of the form of these sponges in the fossil state ; 

 but the constituent spicules of many of these sponges are relatively large and robust, 

 so that they are capable of preservation, and they are frequently met with detached 

 and scattered through the rocks. In some instances they are sufficiently numerous 

 to form thin beds, almost exclusively composed of spicules. It is therefoi'e probable 

 that this order of sponges flourished as abundantly in the seas of the Neocomian 

 period as at the present day. 



