18 



Dun'YOSPONOIDiE. 



and internal parts of the skeleton, while in the thin-walled sponges these 

 different parts are necessarily condensed, and probably abbreviated or only 

 partially developed. Thick-walled siliceous sponges are not of infrequent 

 occurrence in the earlier faunas, but all the Dictyospongid^ and their most 

 closely allied groups, possessed a very thin, almost lamellar skeleton in which 

 a distinction between the dermal and gastral surface is not always clearly 

 perceptible. The accompanying figure which is adapted from one by F. E. 

 StHiri.zK, will convey a conception of the disposition of the parts of the 

 skeleton in the thick- walled siliceous sponges. It may be presumed that a 



D/cfyonalia 



Dfrms/ii.- 



FlGURK 3. Diagram of arrangement of the spicules in the HexactinelUda. (Schulze.) 



corresponding relation of parts exists in the thin-walled sponges but it is not 

 often possible among the fossil species to distinguish the precise position of 

 the isolated spicules in the skeleton. The most conspicuous and strongest 

 parts of the skeleton are the long rods or smooth spicules projecting from the 

 surface of the cup. These are known as the lyroHtalm. They may form a root- 

 tuft at the l)ase of the cup {hasalia) as in many species both of fossil and living 

 foiTus, and in such cases they are sometimes furnished ^vith a means of attach- 



