120 BRITISH STROMATOPOROIDS. 



numerous examples of both Aulopora and Syringopora ; and yet I have never found 

 a single example of either " Caunopora " or " Diapora " in it, nor do I know that 

 one has ever been found. 1 Another but not so striking case is that of the Corni- 

 ferous Limestone of North America, in which we find a remarkable profusion of 

 species of Syringopora, and to a less extent of Aulopora, existing with great 

 numbers of Stromatoporoids ; and yet " Caunopora? " and " Diaporo3 " are exceed- 

 ingly rare. 



3. The converse of this also holds good. That is to say, there are strata in 

 which " Caunopora? " and " Diapora? " are very abundant, and in which Aulopora 

 and Syringopora may be very rare. This is most marked in the case of the 

 Devonian Limestones of Devonshire, in which " Caunoporm " are extraordinarily 

 abundant, whereas species of Aulopora or of Syringopora (unless they are supposed 

 to be nearly all " commensals ") are hardly known, and are certainly very rare. A 

 partial explanation of this may doubtless be found in the difficulties which attend 

 the collection of fossils from these strata otherwise than in polished slabs ; but 

 this explanation would not apply to cases like the Devonian Limestones of Gerol- 

 stein, in the Eifel, where " Caunopora?" and " Diaporoe" are very common, whilst 

 Aulopora? are not particularly abundant, and Syringopora?, if they occur at all, are 

 extraordinarily rare. 



4. If we accept the theory of the commensalism of " Caunopora" and " Diapora" 

 we must suppose that the production of the fossils so named involves something 

 very much more than mere envelopment. Perhaps all the forms of the Stromato- 

 poroids — save such abnormal types as Amphipora and Beatricea — occur occasionally 

 encrusting or enveloping foreign organisms. We should therefore expect that any 

 type of the Stromatoporoids might sometimes be found in the " Caunopora-state." 

 On the contrary, it is only the Stromatoporoids of one particular group which seem 

 habitually to give rise to "Caunopora?" and " Diapora? ;" and it is only certain 

 species in this group which appear to do so. Moreover, the forms which do produce 

 such colonies are mostly non-encrusting types, furnished with a basal epitheca. 



5. Moreover, supposing that " Caunopora?" and " Diapora?" are the result of 

 the associated growth of a Stromatoporoid and a Coral, there are no Palaeozoic 

 Corals which have even a general correspondence as regards their internal structure 

 with the " tubes " of these fossils, except the Auloporoid and Syringoporoid Corals. 



1 As before mentioned, I have recently found a number of specimens of " Caunopora " in the 

 Upper Silurian Limestones of Oesel, these being the only Silurian " CJaunoporcs" that I have ever seen. 

 At one locality (Kattri-pank) these " Caunoporce" are associated with numerous examples of Syringopora 

 hifurcata, Lonsd. (= S. reticulata, His.) ; and I thought at first that the former might easily prove 

 to be merely colonies of the latter living commensally with Stromatoporoid colonies. A microscopic 

 examination of both, however, has satisfied me that in this particular instance the embedded tubes of 

 the " Caunoporce' 1 '' are certainly not referable to this particular species of Syringopora, as they differ 

 from the latter both in size and in their internal structure. 



