254 BRITISH JURASSIC SPONGES. 
It is somewhat difficult to institute a comparison between the Jurassic sponges 
in our British area and those from the Paleozoic rocks previously described, 
since so many of these latter are only known from the isolated spicules, whereas 
in the majority of the Jurassic sponges, the form and the connected skeletal 
structures are sufficiently preserved to allow their systematic position to be 
satisfactorily determined. There can hardly be any doubt, however, that the 
Tetractinellid genera, Geodites and Pachastrella, are common to both divisions ; 
but apart from these the Jurassic sponges appear to be generically distinct from 
the Palwozoic forms. The structure of the Jurassic sponges is, moreover, of the 
same character as that of their successors in the Cretaceous rocks and in existing 
seas; there is, further, a marked absence of those peculiar types of spicular 
structure common in the older rocks. 
As regards the Siliceous sponges, the number of genera and species in the 
British area falls far short of those present in the various zones of the Jurassic 
“ Spongitenkalk ” of South Germany and Switzerland ; but, on the other hand, the 
Calcisponges are probably more numerous and varied in the British Jurassic rocks 
than in those of the Continent. 
