PART II. 

 SPONGES OF THE PALEOZOIC GROUP. 



PRELIMINARY REMARKS. 



Up to a comparatively recent period so few genuine fossil Sponges were known 

 from the older stratified rocks of the British area, that it would have been super- 

 fluous to devote a separate portion of a Monograph especially to their consideration. 

 Even now the number of species is very limited in comparison with those 

 from the Mesozoic strata, and their state of preservation is very unfavorable ; 

 but recent discoveries show that Sponges as a group flourished to such an extent 

 in certain epochs of the Palseozoic era as to form by their remains massive beds 

 of rock of considerable thickness, and, measured by this scale, they were then 

 more numerous than at any subsequent geological period. 



In view of this enormous development of Sponge-life, it is reasonable to 

 suppose that the small number of species known hitherto falls far short of those 

 which then existed. In no group of organisms with structures capable of 

 preservation have the influences of fossilisation acted with more destructive effect 

 than upon fossil Sponges ; at all events, on those in the Palaeozoic rocks ; for 

 thick beds occur built up almost exclusively of their remains, and yet not a single 

 entire individual has been preserved in them ! Their elaborate skeletal tissues 

 have been altogether disintegrated, and only their minute, microscopical, spicular 

 elements, heterogeneously mingled and cemented together into hard rock, remain 

 for examination. 



Under more favorable conditions entire Sponges and fragmentary portions 

 of the connected skeleton are occasionally met with, but at the best the materials 

 for the classification and description of these organisms are very imperfect and 

 unsatisfactory, and such as would be rejected by the student of the living forms 

 as altogether insufficient to furnish generic or specific characters. But since more 

 perfect materials are not available, the palaeontologist is compelled to make the 

 most of those which are at his disposal. It will readily be conceded that the 



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