90 



ORGANIC DEPENDENCE AND DISEASE 



Figs. 74, 75. The perforating sponge Topsentia devonica in a Middle Devonian 

 Stromatopora (Iowa). Fig. 74 is a polished section; Fig. 75 shows the embedded 

 sponge by transmitted light. 



these made by the worms are in contrast to the perforating 

 tubes described because of their usual simplicity and their 

 greater size, and among the fossil occurrences these fea- 

 tures lead to comparatively easy and safe recognition. 



These simple worm-borings are found in many sorts of 

 solid calcareous organic masses in the Paleozoic rocks. 

 While we find in the Ordovician worms growing concur- 

 rently with solid corals or coralloid bryozoa, like Praso- 

 pora, there is no present evidence of perforating worms 

 boring into such masses and more specially into molluscan 

 and other heavy shells, until late in the Silurian, from which 

 date they acquire greater abundance and in the Devonian 

 faunas become widespread. As the evidence now stands 

 they were rife in the early Devonian everywhere, even in 

 the austral faunas of this period which are in many respects 

 widely distinct from the contemporary faunas of the north. 1 



i See Clarke : ' ' Fosseis Devonianos. ' ' 



