FOSSIL SPONGES, ETC. 13 



may be regarded as a characteristic lithistid sponge. 

 Fossil sponges are always commonest in limy and 

 arenaceous rocks. 



As regards the habits of recent sponges, it is as 

 well to be acquainted with them, for they throw light 

 upon the conditions of ancient sea-beds, inasmuch as 

 we find that modern sponges are characterized by the 

 same habits as the oldest kinds. In this way a 

 knowledge of recent and fossil forms is reciprocal in 

 its illustrative effect Thus the genus Cliona a 

 remarkable sponge which has the habit of boring into 

 the denser structure of bivalve shells, as may be seen 

 in a thick-shelled, deep-sea oyster on the nearest fish- 

 stall has indulged in this habit ever since the Silurian 

 period. Shells of fossil Pterinea, in the Malvern beds, 

 are frequently found perforated by ancient burrowing 

 sponges ; and in the chalk near Norwich we find 

 the solid cones of Belemnites (internal bones of ancient 

 cuttle-fishes) riddled through and through by boring 

 sponges, which have thus left a very delicate but 

 graceful pattern of the walls of the burrows. Again, 

 in our British seas particularly, we have a large 

 number of existing sponges encrusting or growing 

 on other natural objects ; and we find fossil sponges 

 such as Sparsispongia which adopted this habit as far 

 back as the Silurian times. 



It is probable that in the earlier seas of the globe 

 the divisions of the lower marine animals were not 

 so distinctly marked off from each other as they are 



